A caution about Plex - it is dogshit on mobile. As in, won't play a song all the way through. As in, stops randomly. You can try and jog with it but it won't last a mile. It's amazing.
And Plex does not store your files anywhere you can get to it without jailbreaking your phone. And once you find those files, they are assigned garbage hash-speak filenames. So even if you wanted to do something like sync via Plex and playback via Doubletwist or something? yeah no. not so much.
I achieved an uneasy peace with Google Music because T-mobile doesn't hit me for streaming that so I play whatever i want. Often, it fails to find the song on the first, second, third, fourth, fifth or sixth time but once you've pounded it a few times, it comes around. However, when I switched to Fi I found myself pretty much fucked unless I was willing to eat the data from running.
I have griped to Plex about this, as have many others. For a company that really wants you to believe they're serious about music, they're stunningly unserious about music. I mean, how hard is it to, you know, play a fucking MP3 file?