I've tried both and... don't actually hear much of a material difference between the two. If I understand correctly, Apple's using their old standby of 256Kbps AAC and Spotify uses 320Kbps Vorbis, so Spotify is slightly better on the technical merits -- but they're both at sufficiently high quality that audio nerds would be demanding anyone who claims a definitive difference do some blind ABX testing to prove it.
I ended up sticking with Apple Music after my Spotify Premium trial ran out, but I suspect that's largely because it's what I'm used to. I've seen lots of "Apple Music's recommendation engine is crap," but that's not actually been my experience -- their "New Music Mix" isn't great, but the whole of their "For You" page almost always has stuff I'm interested in and I really like digging through their curated playlists. It's been a much more rewarding service for just digging around exploring in than either Spotify or Tidal have been for me. (Although Tidal undoubtedly has the best sound quality if you're willing to pony up for the lossless/MQA tier.)
256Kbps AAC and 320Kbps Vorbis both comfortably exceed the threshold of audibility for 99.999% of people 99.999% of the time. If you somehow cared about the difference between either of these (or uncompressed, for that matter) then you're not listening to music properly.
The days of 128 Kbps MP3s encoded with L3enc or Xing with horrific pre-ringing and squelched cymbals is long gone.
I ended up sticking with Apple Music after my Spotify Premium trial ran out, but I suspect that's largely because it's what I'm used to. I've seen lots of "Apple Music's recommendation engine is crap," but that's not actually been my experience -- their "New Music Mix" isn't great, but the whole of their "For You" page almost always has stuff I'm interested in and I really like digging through their curated playlists. It's been a much more rewarding service for just digging around exploring in than either Spotify or Tidal have been for me. (Although Tidal undoubtedly has the best sound quality if you're willing to pony up for the lossless/MQA tier.)