You've got a tough problem - I've had both and liked them both. In some ways, I think that both are evolutionary improvements on the Glock. The M&P *may* incorporate some lessons learned from the Walther P99, but that's speculation. I don't think you would go wrong with either of them. Both companies seem to have reasonable support.
About the only thing I can do is point out feature differences:
M&P - easily interchangeable grip panels
XP - grip safety