Because the previous built in music player ran so flawlessly I would always prefer to use that, but since that got absolutely shafted on 8.1 I used Mix Radio for a while, which actually highlighted how good the app had been since a few upgrades, but as has been mentioned it would take a while to load the library so I was kinda caught between two music apps not working particularly well with 8.1…
Well I just had an update to Nokia mix radio today and the library now opens up instantly pretty much as before the 8.1 update so I think I'm going to use this from now on; unless they really sort out the Xbox music app.
But as I said my main reason for using the Xbox music app was because being baked into the OS it ran so quickly and flawlessly, now that it's in theory just another app there's little reason it would be any better to use this over Nokia mix radio. I also prefer the Nokia smiley face headphones on my start screen as opposed to the Xbox headphones.