As someone who's never used Spotify I guess I don't know what I'm missing out on but I found it interesting all the same to compare them. It's just more economical to have YouTube premium with YouTube music added in for free!
I've mostly used YouTube Music, though my friends use Spotify, and I think the addition of being able to watch/listen to YouTube videos in Music is huge. Spotify is great if you only listen to label-published artists, but when you want to include tracks and mixes from indies, YTM is just the only way to go