I've always used Spotify and plan to use it forever. I like it mainly because of the playlists (especially Discover Weekly) and the social aspect of it (I can see what my friends are listening to, we can make collaborative playlists, etc)
I really liked Spotify for the playlists they offered, but I'm sticking with Apple Music. I like how it's integrated with my iPhone, and they are starting to provide better playlists for users. I think the UI on Apple Music looks much better than that of Spotify on my iPhone, and I like that new releases are available hours before they're available on Spotify. Apple Music has already gained 11 million paying customers in a little more than 6 months, and I expect the app to get even better.
Spotify seems to be more stable for me, but I much prefer Apple Music. There are so many great artists and musicians who refuse to upload their complete catalogs to Spotify, Peter Gabriel and Andrea Bocelli being two examples. Meanwhile, Apple Music seems to have everything except T.F.L.O.P.