I'll give you that it's baked into the phone and can be used with Siri. However, you cannot handoff music at all, and with Spotify I can control the music playing from my Mac with my phone, my watch, etc. Spotify Connect. Allows you to hand off music from any device seamlessly, and control music playback from other devices, not just apple products. And it works insanely well.I don’t know how when Apple Music is baked into most Apple hardware such as the iPhone. Maybe your use case is different, I just find Apple Music to be much easier, if I want to use Siri to select a song or playlist it works well.
So, Spotify works *between* devices much better than Apple Music does, and Apple Music works more integrated with a single device.
I use Spotify Connect between my MacBook Pro, and my iPhone all the time, and control music playback on my MacBook with my Apple Watch. None of that is possible at all with Apple Music.