Spotify:
+ Huge catalogue
+ Great Playlist selections, and they're constanly updated
+ Nice, clean interface
+ 320 kpbs streaming
+ Dedicated Desktop app
+ Leader of the market, so better community support and community features
+ Easier to find new artists
- Desktop app can be kinda buggy and slow
- No way to sync your own music, unless they're on Spotify's catalogue. (lots of omissions)
- No FLAC compatibility
Play Music
+ Upload ANY music, even if not on their catalogue, and listen it anywhere.
+ FLAC uploading support (even though they are converted to 320 kpbs mp3 files)
+ If you REALLY LOVE Taylor Swift

P)
- Horrible, slow and buggy PC (webapp) and mobile apps
- Since its marketshare is small, it is often forgotten by artists (they still appear on the service, but takes a little longer)
- Bad playlist selections
- Harder to discover new artists.
That said, I chose Play Music only because of the uploading feature, as it is really important for me (I listen to lots of gaming/youtube/cover/brazilian content, there's no way to listen to them via Spotify). As soon as Spotify offer this feature I'll be moving over.