I don't have experience with either. But the Mossberg looks really............weird.
The nice thing about the M&P15-22 is that almost any external AR15 parts are interchangeable. Grips, stocks, rails, etc. The Mossberg is just an AR "style" (hardly) rifle. If you're getting the Mossberg, might as well get a 10/22 and put on an AR type stock.
M&P15-22 is the way to go.