main difference that I'm aware of is the interchangeable grip, and the higher capacity of the M models, plus the M has a match barrel, not sure about the xd. but I don't have the normal xd so that's as much as i can help
I had a XD just picked up my XDM yesterday it's like a different world the XD is a great gun but the XDM is so much of an inprovement. I think springfield learned from the XD an made it better unlike the other companys *cough*glock*cough* have taken years and generations to make any changes.
So my choice is the XDM.
(and you know I'm not a glock hater just about time for a change aka 4gen)
My xdm had a tad longer barrel than my sis's xd too. I think it may even have been a wee bit wider too. I loved my xdm and had I not found the marlin for sale that I had to have, I would still have the xdm. I'm tryin to buy it back from the guy I sold it to tho! Lol. I miss that pistole!