This is purely anecdotal of course, but the libraries are similar or better for what I've been listening to. Especially asian music seems to be easier to find on Google music.
The mobile app for android is good, but I haven't seen it receive many updates this last year. It seems like it is pushed a lot stronger in the US. They even give you Youtube Red (the no-ads/music/support creators-thing) with the same subscription. [1]
I liked that they allow storage of up to 20 000 of your local songs (if you still have those), integrated into your online library.
I used a fan-made application to recreate my playlists from spotify, but it required that I was a premium member on both services at once, and just directly used my credentials. Most playlists were kinda butchered. I don't have a good migration-strategy to recommend, I'm afraid.
EDIT:
Downsides: It does not have an official desktop-app, you have to use the browser.
Does not have official support for last.fm
1: https://play.google.com/store/apps/details?id=com.google.and...
