They're very comparable (I'm a paid user of both). I've discovered realms of amazing music through the recommendation engines of both of them.
I am a huge Apple fan, but Spotify is better in so many ways (Discover weekly and other lists like “piano ballads” - which while not perfect are better than I get on Apple Music)
Maybe it’s because my wife and I share music lists or something but the recommendation engine in Apple Music is not good for me.