The grey interface is a downside for me (also behind the menu's to select music, which cannot be changed through MPAtool). In addition, the slider of the new Xbox app is handy and it is supported with updates. For example, we'll be getting a live tile sometime soon. But to me, the black background in the whole app will be the main reason to go to Xbox Music once it's "optimal" and has a transparent tile.
But for now, the old app offers speed, though there's a latency compared 8.0 when you open the app and when you press "albums" (or artists, whichever you choose).