Basically they did a minor redesign on the "M", they made the bbl 4.5" long as opposed to the 5" Tactical or the 4" Service, improved the trigger (I still think the reset could be better), added a removable back strap, changed the grip texture, put on different sights (the rear notch needs to be wider), and made it able to be field stripped without pulling the trigger. SA bills the barrel as being "Match Quality" but unless you're a much better shooter than I, it doesn't make a difference. They also re-profiled the slide to make it more physically appealing and improved the cocking serrations and re-designed their polymer in order to make the grip thinner allowing for a redesigned mag that holds more ammunition.
All that info (and some other changes) can be found on Springfield's Website...
I really like the gun, I was never a fan of the original XD for a couple difference reasons, I didn't like the cocking serrations, this is important to me as I have overly sweaty hands and I need to get a good grip. On a Bi-tone XD, I could hardly cycle the slide period. I was never happy with the balance of the 5" or 4" XD, but the 4.5" just feels better to me.
All those other changes don't really matter to me. I'll probably change out the sights, have the trigger re-worked a bit, I'd like to take belt sander to the grip and remove the "mega-lock texture"....
Or I could just leave it alone and still be content with it.
This was a brief range report Springfiled XDM-9 Range report, I took it back out yesterday and put about another 150 rounds through it and the more I shoot it, the more I like it.
But I still think the rear sight notch needs to be wider.
As much as I'd like to go nuts on the gun and make it 100% to my liking, in all honesty I'll probably just change out the sights and call it a day.