This was a question surrounding in my head during more than 2 months. Now, I think I have a resolution for it. None of them is better than the other, but definitely they are designed for two different kind of users.

Quality of sound

This is a very touchy topic. In my opinion, Deezer offers better quality of sound than Spotify. Period. I’d like to stress that I’m talking about the two premium offers of them, because in Spotify you cannot have HQ in the free offer. I suggest you to uncheck the “Set the same volume level for all tracks” in the Spotify Desktop client in order of getting a better quality of sound.

Clients

This is the point why I think both applications are for two different kind of users. I love clients for applications. In fact, I use thunderbird for checking my mail, Adium or Pidgin for IM, and so on. I just love Spotify Client. It’s very easy to use, and very well designed in my humble opinion (I talk in terms of usability for the user). You also can use hotkeys with it, and that’s a killing feature. On the contrary, Deezer only offers for the moment a web application which I found very old fashioned, and where you don’t have hotkey functions. However, both clients offers “Offline mode”, and that’s OK. Spotify in this regard was more intelligent than Deezer. Why? Spotify has a desktop client that works in Linux, Mac and Windows because maybe they knew that doing a webapp wasn’t going to be that hard at all. In fact, now Spotify is offering a web service that is looking great.

But not everything is perfect with Spotify client for mobiles. In this zone is where Deezer just rocks. In Deezer, you can search for an artist, and if you like it a lot, you can just touch the “Offline mode button”, and automatically the music gets in your device. Spotify does not work like this… In Spotify you have to connect your phone to the same wi-fi network as your desktop client in order to sync the device.
Update: I think I was wrong. After playing a bit more with the Spotify’s mobile client, I realized that one can “cache” the music using either WIFI or 2G/3G/4G connection.

Local media

Spotify client let you play music in your local library. And this is very nice, knowing that you cannot find all the artists you like yet. Deezer lacks of this option, and instead you have to upload your music to their servers in order to have access…

Activity, applications,  and sharing

I think in this one Spotify is best. I have more friends in Deezer, but most of them are inactive,  and it is the contrary in Spotify.  In Spotify I have more active friends. Sharing is OK in both, but I rather like the way you can share your music in Spotify. Both offers connection to your last.fm account, and they have applications that let you discover new music (even the one from last.fm recommendations). In my opinion, Spotify apps are way better than in Deezer. That is because of the fact in Spotify they run within the client, and they look very nice (mostly I like the fact that in Spotify apps provide more information than in Deezer, and that is because in the latter one, they are a web app).

Radio

I think there is not too much to say in this one. Spotify rocks in this.

So, Deezer or Spotify?

This is a question of what you like, because both services have the same cost. If you like clients, using your hotkeys, and accessing to your local files, so Spotify is your way to go. If you don’t care about that, but you are looking for a better quality of sound and you love syncing your phone, so use Deezer. The difference in quality is not that strong, I mean, it’s not a big gap, but still I can hear it. I prefer Spotify, and now they have a web app as well. The lack of web app was an argument from Deezer users. I have to admit that I miss the way in which Deezer downloaded the music in the mobile app though. I also like more the Spotify’s design than Deezer (yes, that ‘s not a good argument at all :)).