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While digital music downloads are on the decline, music streaming services are soaring in popularity, enabling you to pay a small fee each month, or even get the music for free if you can put up with advertising, and get access to huge music libraries to listen to whenever and wherever you fancy.

Spotify seems to be leading the pack when it comes to music streaming in the UK, but there are several worthy alternatives, including Deezer, which is growing in popularity. Plus, Google has launched a new service that could scare rivals. Here, we put Spotify, Deezer and YouTube Music Key head-to-head to see how they compare. See also: Spotify alternatives

Worldwide, Spotify has 50 million users, with 12.5 million of those paying for the ad-free version, while Deezer has 16 million users, with six million of those coughing up cash.

YouTube Music Key launched in November, and is currently still in beta. In the UK, you can register your interest on the YouTube Music Key site, but you won't get access until the service becomes more widely available early next year. Alternatively, you can subscribe to Google Music Play, which will get you access to YouTube Music Key beta.

Deezer vs Spotify vs YouTube Music Key comparison: Music library

The most important element of a music streaming service is the music, right? So we'll start by comparing the music libraries of Deezer, Spotify and YouTube Music Key. Each has millions of songs available to listen to – and in the case of YouTube Music Key, watch videos for.

Spotify has 30 million songs in its library, which reaches 58 countries, while Deezer has 35 million songs that reach more than 180 countries.The amount of songs available to each country varies, so it's likely that the reason for the higher song-count in Deezer's library is local music not available for the UK market.

We did find, though, that Deezer presented some extras for some of the artists we searched for, particularly offering live albums that Spotify seemed to lack.

Google states that YouTube Music Key has "millions" of songs, but we've not been able to confirm exactly how many millions there are. Some songs without videos are available to listen to with a static image instead, but in the case of some artists (including Taylor Swift who retracted her songs from Spotify recently) only the singles released with videos can be found.

YouTube Music Key is currently available in beta form in the UK, US, Italy, Spain, Portugal, Finland and Ireland to those who've been invited and Google Play Music subscribers.

Deezer, Spotify and YouTube delivered perfectly satisfactory results for every musician we searched for, so we don't think you'll be disappointed any of these services.

Plus, more songs are being added to all three services every day, so if there's something missing from the library that you were hoping to listen to, chances are it'll appear at some point in the near future.

You'll be able to add the songs you find and like to your own playlists and favourites if you choose to in Deezer, YouTube Music Key and Spotify.

Deezer vs Spotify vs YouTube Music Key comparison:Design and user interface

One of the big differences between the three streaming services we're comparing here is the design and user interface, and we think we prefer Deezer overall.

We like Deezer's light, clear and simple design across both the web/desktop versions of the service and the mobile/tablet versions, and think it's easier to navigate than the dark, quite complex interface of Spotify.

YouTube Music Key will appeal to those already fond of and familiar with YouTube, as it uses the normal YouTube app. In its early beta form, it's only available as part of the Android app and website, but an iOS app is coming soon.

Not every video/song is licensed under Music Key, but those that are will be ad-free (it's the blue Ad-free icon that indicates that the video/song is part of the service), and available to listen to on your phone or tablet even when the screen is switched off. You can pause, play and skip tracks from the lock screen, too.

We found that we were able to get stuck in immediately with Deezer and YouTube Music Key, but Spotify took us a bit longer to figure out. Once, we had sussed it, however, the three services became equally enjoyable to use.

Deezer vs Spotify vs YouTube Music Key comparison:Music discovery

Another big appeal of music streaming services is the discovery features. Music lovers are always looking for something new to listen to, and Deezer, Spotify and YouTube Music Key offer up recommendations for their users.

Deezer has a cool feature called Hear This (shown above), a music feed that's personalised for you. It shows you new albums and tracks from artists you listen to or add to your favourites, as well as Deezer picks based on your listening habits and playlists created by users with similar music tastes to you, so you'll never run out of tracks to listen to.

We prefer Here This to Spotify's 'Discover' feature (shown below), which we found to be a bit hit and miss. Plus, Spotify doesn't recommend playlists in the Discover section, just artists, albums and individual tracks.

Deezer offers up 'Similar Artists' for each artist you search for, too, much like Spotify does with its 'Related Artists' tabs. Both services recommended largely the same artists during our tests.

Where we found Spotify to be better for music discovery, though, is in the Browse section. It has a much easier-to-use interface than the rest of the app, too. You'll be offered an overview, top lists, genres and moods, new releases and news in the 'Browse' section of Spotify. The genres and moods feature is particularly good, offering playlists that fall under categories like "Focus" for revision sessions or "Chill" when you're having a relaxing evening in.

Deezer's 'Explore' offering is similar to Spotify's Browse, but doesn't have quite as many options to help you find what you might be looking for. However, new 'Labels' and 'Eras' sections are coming soon so improvements can be expected.

YouTube Music Key is a bit different, in that its discovery tools are more simple and limited than Deezer and Spotify's. As it uses the normal YouTube app/website, you'll see suggestions based on videos you've previously watched and songs you've listened to.

However, it's important to remember that YouTube Music Key subscribers also get access to Google Play Music (and vice versa), which is a streaming service more reminiscent of Spotify and Deezer and therefore offers better discovery options. Find out more in our Spotify vs Google Play Music comparison.

Deezer vs Spotify vs YouTube Music Key comparison:Radio

Deezer and Spotify both offer Radio services in addition to the music streaming features, and YouTube Music Key's link to Google Play Music means subscribers to the Google service will get radio too.

You'll find themed radio channels, partner radio channels and artist radio channels in Deezer, and lots of different genres available to choose from. The artist channels, for example, intelligently picks songs from the artist you've selected and similar or related artists. You'll get a limited amount of skips in the Radio feature on Deezer if you're using the free version, though.

Spotify's Radio feature is very similar to Deezer's offering up Artist Radio for the artists you like in the same way. There are also lots of genre stations to choose from. Spotify doesn't set a limit on the amount of skips you're allowed on each channel, though, so if you don't like a song that's playing you'll be able to skip right ahead to the next one, or the next one, and so on.

As mentioned above, YouTube Music Key itself doesn't offer radio options, but by using Google Play Music which comes included as part of the subscription fee, you'll be able to create your own radio stations based on your favourite artist, album, song or genre with unlimited skips.

Deezer vs Spotify vs YouTube Music Key comparison: Offline watching

Helpfully, all three of these music streaming services have offline options. Many YouTube Music Key videos and songs are available to download for watching and listening to offline.

Spotify Premium subscribers get the ability to download music and listen offline in high-quality, limited to 3,333 tracks that'll be available to listen to offline for up to 30 days.

You can sync albums and playlists to your device in Deezer Premium for listening to offline, too.

Deezer vs Spotify vs YouTube Music Key comparison:Other features

Deezer, Spotify and YouTube Music Key also offer some extra features that we've not yet covered, and these features differ between the three services.

We love Spotify's 'On Tour' feature (above) which means you'll know if an artist you've searched for is playing a concert in your country any time soon. There's also a merchandise feature in Spotify, meaning you can find T-Shirts, badges, mugs and more from your favourite bands and musicians.

Social integration with Facebook is present across both Spotify and Deezer, allowing you to see what your friends are listening to. In this instance, Spotify has a one up on Deezer thanks to the bigger listener base, which makes it more likely that more of your friends will be subscribers.

Both Spotify and Deezer have Apps available to bring extra functionality to the service. Deezer's App Studio has more than 100 apps for anything from lyrics to chord extraction, while Spotify also has apps available in its App Finder feature on the desktop app. The apps available are very similar here, too.

YouTube Music Key's pairing with Google Play Music means you'll also be able to add up to 20,000 purchased songs to listen to through the service, even if those songs are from iTunes.

Deezer vs Spotify vs YouTube Music Key comparison: Price

Spotify and Deezer both offer free, ad-supported versions in addition to paid versions, while YouTube Music Key is only free as a month-long trial before you'll need to pay for a subscription to the service.

With the free version of Spotify, you'll get access to the entire library on mobile, tablet and computer, but you'll hear ads regularly between tracks. Plus, you'll only be able to listen to audio at a quality of 96kbit/s, which is quite poor compared with the 320kbit/s available at the top end for Premium subscribers.

Upgrading to the £9.99 per month Premium Spotify subscription will let you download music to listen to offline, and you won't hear any ads between tracks for uninterrupted listening either.

Spotify is currently offers a one month free trial of the Premium version, so you can see whether you think it's worth the £9.99 before you begin coughing up cash.

Meanwhile, Deezer's free version is not quite as good as the free version of Spotify, though a recent update means you'll now get access to the service (though it is limited) on mobile or tablet, in addition to the web. It'll come complete with ads, and is low quality audio like the free version of Spotify.

Deezer then offers an option for subscribers willing to part with their money. Deezer's Premium option will set you back £9.99 per month. With Premium , you'll get access on the web, mobile and tablet, offline options, unlimited streaming with no ads and better sound quality up to 320kbit/s.

Spotify and Deezer are both available on a huge range of devices including Android, iOS, Windows, Windows Phone and OS X.

YouTube Music Key is currently limited to Android and web browsers, but is coming to iOS soon. It's also priced at £9.99 per month, with access to Google Play Music included in that subscription price.

Overall, Spotify, Deezer and YouTube Music Key are all excellent music streaming services, and if you've never tried a similar service before you should definitely give one a go to see whether you like this new way of listening to,sharingand discovering music.

We prefer the design and interface of Deezer, but once you're used to it, Spotify's interface is good, too, and YouTube's is familiar although we're still not fond of it.

We also prefer Deezer's 'Hear This' feature to Spotify's Discover feature, and feel that YouTube's discovery feature is quite limited in comparison unless you hop over to Google Play Music.

But Spotify's Radio feature is better than Deezer's and YouTube Music Key only has radio as part of Google Play Music. The free version of Spotify will serve you better than the free version of Deezer will (there's no free version for YouTube Music Key users).

Plus, the number of subscribers to Spotify higher than the other two services listed here and social integration is great, so if you're planning on sharing the experience with friends, Spotify might be the better option.

Of course, YouTube Music Key is the only service that offers video as part of the package, so if that's something that appeals to you it's probably the one to go for, particularly as Google Play Music comes bundled in the same subscription fee (good move Google). You can find out more about YouTube Music Key here.

Buying Advice

We think it's worth giving all three of these services a go with the free trial before subscribing, as it's really down to personal preference which one you'll prefer. They have many, many similarities so it's those small differences that'll help you make the decision between the three.