My two cents:
I used to have Spotify but moved to Google Music for the simple reason that it allows you to upload music to your account and stream it wherever you want, which allows me to listen to King Crimson, Transatlantic, TNMB, etc, when I'm at work. It also lets you download things to your device and listen offline.
Regarding music availability, there are a few things that are available in GM that are not in Spotify and the other way around, though there's no real big difference.
What I really miss about Spotify is the weekly recommendation playlist it builds based on your listening habits. It is a great source for discovering new music. GM has something akin to that however their algorithm is not nearly as good.
Apple music I haven't tried and won't since I don't own apple devices.