TexasCHLforum.com

Why the XDm is "Better" than the XD:
First off the barrel is supposedly match grade.
The trigger is supposedly better (I still recommend a trigger job on the majority of XDm pistols).
You have interchangeable backstraps to better fit your hand.
On the 9mm and .40S&W models you get more ammo capacity (.45 ACP is the same).
Take down doesn't require a trigger squeeze.
The XDm (9 and 40)is available in 3.8 and 4.5 inch barrel lengths while the XD is available in 3, 4, and 5 inch lengths.
The XDm now has a compact variant (3.8 with shorter grip) in 9mm.
The ergonomics are better tuned.

Overall, if the XD fits your hand well, and you shoot it well why spend the extra money on an XDm?

BTW, I own a 4" XD and a 4.5 inch XDm in 40 as well as a 4.5 inch XDm in 45.