you can set playlists to Offline and download everything while you're on wifi
there are advantages to having mp3s but I like having my entire library available on my computer, phone, tablet, ps4, etc. at any time. When we went on our roadtrip, we made a huge playlist and set it to Offline. no issues. Also it helps that TMo doesn't count Spotify towards data usage. If they counted it, I may think differently lol
But mp3s are free and don't use any data