Price point may be the largest distinguishing factor on these two pistols. I own the M&P45, and I've fondled an HK45. The HK is a very nice gun. Both pistols have, IMO, advanced ergonomics. The HK has the added ability to actually change the side panels of the grip (in addition to the palm swell).
At my LGS, the HK45 was $950. Whereas I picked up my M&P45 (DBE w/safety) for $499 ($449 after rebate).
Sorry I can't add more. They both feel great in the hand. I'll add an HK to my selection at some time in the future.