I have both a M&P40c and a XD40sc. The M&P definitely fits the hand better and I like the feel of the trigger surface better BUT...
MY XD is more accurate and I like the tacile feel of the loaded and cocked indicators for night use (try looking down the M&P loaded indicator hole in the dark). The grip safety is also comforting (why do people complain about it on an XD but never on a 1911 or old centennial... huh? huh?). If it makes any difference, the XD mag release is ambidextrous as-is w/o taking the gun apart to change it.
Like others said, rent or borrow one and try it then decide... I'm still trying to make up my mind.