In terms of Web Player
Deezer: Fast and responsive
Spotify: Slow, loads and skips queues.
In terms of smartphone app (on Windows Phone)
Deezer: Piece of shit, loading forever.
Spotify: Light and intuitive
In terms of sound quality
Deezer: 128kbps or 384kbps in MP3, no audible artifacts.
Spotify: 96kbps, 160kbps or 384kbps in OGG. At 96kbps and 160kbps, for some reason there was excessive hissing sound for quieter songs and at quiet passages, such as at the end of a track.
In terms of music library availability
Deezer = Spotify (at least from what I can see, tested Spotify against my Deezer’s playlist)
There you go!