I was a huge XD/XDM fan for years and I have way too many of them. Last year I tried an M&P and absolutely loved it. It was good enough that my wife also switched to an M&P from her Glock.
Here are the reasons I both like the M&P over the XD/XDM:
- The grip safety on the XD was never an issue for me since I shoot so much; but, it does cause problems for new shooters. I loaned an XDM to family members twice for IDPA matches and they both ended up with the slide frozen in a position where we really had to force it to get it unstuck...
- The match grade barrel is nice on the XDM, but I can shoot the same group sizes with either an XDM or a standard M&P. In either case, I can consistently put 10 rounds into a 2" circle at 15 yards (when I'm trying for accuracy).
- The barrel on XD/XDM sits noticeably higher than the M&P. That makes the XD/XDM feel top-heavy and also makes the M&P quite a bit faster for follow-up shots. The XD/XDM can be just as fast with practice, but the M&P seems to do it more naturally (for me at least).
- The M&Ps are quite a bit cheaper than the XDM.
The main problem with the M&P is the trigger - it's honestly one of the worst triggers I've ever felt. Fortunately, it's easy to swap it out for the Apex DCAEK trigger and that helps a lot. I've found that I like the standard/duty Apex trigger better than the competition version since there seems to be less take-up; but, the competition version is lighter and has a shorter reset...
Anyway, I probably blabbed on way too long. You won't go wrong with either one - but I, personally, prefer the M&P.