It mostly boils down to the algorithm for me.

I tried YouTube music for a few months and it felt much more spaghetti-at-the-wall than Spotify.

My opinion is that this is because of the way that Spotify applies algorithm and curation in a highly unified way.

Spotify Radio, Release Radar and Discovery playlist are just wonderful. Right down to the ordering of the lists it accounts for broad tastes, current personal trends, building accessible gateways to new genres and so on.

YouTube Music felt a bit like the video algorithm. "You watched a Minecraft video? Prepare for the avalanche of tenuously related but quite obnoxious Minecraft content"