I use YouTube because its free to search for new music, and has most/everything I listen too. I do like Spotify though, but I don't want to pay for it, and the ads annoy me on the free version, so I don't use it.
I'm a Spotify loyalist to the core. The discovery tools gets to me most times. It helps in finding artists and tracks I never came across with before. It has also a very friendly user interface that does go well with the smart phones
My Apple Music free trial ended, so I'm using Spotify now. Between the two, I prefer Apple Music. The audio quality is outstanding. I enjoy YouTube Music, but I don't use it frequently.