Google Play Music has Jay-Z, as far as my research shows. (unfortunately I can't check as I have Spotify due to their network affect roping me in through a family plan) Is it really fair to say Spotify has a bigger library outright?
Spotify claims to have 35m songs and GPM 40m. Not that that means much, either. The killer feature GPM has over Spotify is that you can upload your own tracks to fill the gaps wherever their library is lacking.
Spotify has Jay-Z as well.[0] I would say it's hard to really quantify which library has the larger selection as most of the differences would be completely obscure music that no one listens to. However, I haven't seen any artist on GPM that isn't on Spotify while the opposite can't be said.
Spotify has the ability to upload music as well, however that is contingent on you having premium.[1]
I'm not going to dig around for metrics on this, but my intuition is that Google covers a larger base of "established" musicians whereas Spotify seems to be dominating new releases from smaller artists.
Google Music, Pandora, iTunes, are seemingly there to cover bases. The momentum appears to be towards releasing your music on Spotify first for your friends to listen to.
Yeah I like the social feed. Wish you could get to it on mobile though.