I've found that they generally match one another feature-for-feature. I like the time- and activity-based stations for Google ("You're driving home on a Friday night, here's some up-beat indy house with female vocals"). I found Spotify tended to repeat itself too often, and music discovery wasn't as easy for me. I've also not used it in over a year, and I know they iterate rapidly.
I also like that with Google Music, you can upload up to 50,000 tracks of your own and they get included in radio stations, playlists, etc.
Both have similar offline functionality (you can even take a few hours of a radio station offline). Riding BART with intermittent cell data on the way to work hasn't ever been an issue for me, because it pre-fetches a few songs at a time when signal is good. AndroidWear and Sonos integration (two techs that I use regularly) are excellent with Google, and I know Spotify is on Sonos but I haven't tried their mobile app w/ Android Wear.
Best part, if you have a Chromecast at all, you can go to http://chromecast.com/offers
and get a 90 day trial.