i'm a dino, i held out til now but i can't resist any longer

after a cursory google, i'm leading toward google play music - seems like with the option to upload my music (is this any music, or does it have to "match" tracks from their library?) plus the included youtube red subscription, it's the better value

but i know spotify is the ppls choice - what am i missing by forgoing it? also, am i understanding correctly that while you don't upload your own music per se, you can have it somehow relate to your existing library? can someone explain that to me?

I remember trying google play music at one point and having trouble uploading my files as advertised... any thoughts there? they're in varied formats, including m4a, so i know that could be at issue