I do YT as well, but just because I'm more familiar with it. I know there are ads in both of the free versions so it doesn't matter in that regard, I just don't want to learn how to navigate a new platform.
I don't use either for storage, and didn't even realize that it could be done. I watch short clips on YouTube and I listen to playlists on Spotify, but that's about as exciting as it gets for me.
I don't store music, but I mostly go to Youtube now for their ambiance music. It's calming, and I don't have to listen to ads. There's also the comfort of not having to choose from many options.
The watch later does me good, so I'll have to go with YouTube. I don't often go on organizing a playlist, and having that option saves me from thinking. It's also easy to download anything from there.
There's the algorithm factor that lets YouTube suggest the best stuff. The recommendation is one of its features that makes things easier when listening to music or even a lecture. Then yeah, there are the easy downloads, and you can also skip the ads.