Spotify's discovery features really blow Apple out of the water (my tastes fit very poorly into Apple's curated ecosystem of industry-promoted genres).
Aside from that my roughly 6 month trial of Apple Music confounded me greatly trying to maintain my personally curated library of music separate from the streaming collection. In particular, Apple's metadata has been a disaster and I've extensively retagged my collections to suit my preferences for searching and sorting by genre, eliminating extraneous info from album titles, etc.
Enabling Apple music really screwed that all up royally and I don't want to risk wrecking everything I had to fix by turning it on again.
Aside from that, Spotify still totally wins for me over music discovery, something that will not likely change until Apple allows algorithms to improve discovery, which seems unlikely for the foreseeable future.