I've been discovering and listening to more music since switching from Spotify to Google Play Music. I boil this down to three reasons:
1) The idea of building a Library of music without having to create Playlists. There's a difference, and it matters.
2) Google Play Music makes it significantly simpler to download albums and singles to your device by taking out the playlist step that Spotify forces.
3) The radio on Google Play Music actually plays the song you chose first. This means I actually use it, and discover new music this way.
On the other hand though, Google Play Music's big missing piece is still an iOS app - wtf Google? Close this gap, and there's no debate to be had. Since Google have consistently supported both iOS, Android and Web with almost all major services, we have to wonder what is happening behind the scenes to create this issue.