As to your specific questions, the M&P is too new in .45 to have much of a track history - reliability is still a question mark. It wouldn't surprise me if the pistol will have at least a few teething problems that won't show up until there are quite a few in the hands of the buying public.
XD45 is new, too, and most of them seem to work quite well. But if you look through this and other forums, you'll find a fair number of people have had problems. There are also quite a few used XD45s on the used gun market - not a good sign for such a new pistol. (On the plus side, SA seems to actually fix the ones they get back under warranty.)
Personally, I'd give either one another year or two on the market before taking the plunge myself . . . but then, I'm one of those who got burned on another highly-regarded polymer pistol (Kahr P9) so I may have a bit of a bias against new plastic guns.