You can try each of them out and see what you like about each then decide.
I used Google Play Music before Youtube Red was available.
I imagine they are similar. The only music subscription which I would change to if it survives the next few months is Tidal for lossless but waiting for fiber to my house.
No adverts and there are actually some good shows on Youtube Red, ok there is probably 2 lol but no adverts anywhere.
Music discovery is great, you can start any station from any song, album or artist you want and based on machine learning algorithm populates the station, it's pretty spot on in my own experience and becomes more refined as you like or dislike songs.
Now I've got it to where if I press "play random" I will not need to skip any songs.
Curated playlists are great too activities to geek days to studying, it will quickly start to work out what playlist to offer at what time (like how Google Now / Assistant knows what you need at what time / day / date).
Like I said to start off with though, try both out, you will likely have a 3 month or if you're lucky 6 month trial waiting for Google Play Music and probably at least 1 month for Spotify.