I didn't notice an answer to this yet so here's what I know. The XDm's are upgrades on the XD's in the following ways:
1. Match grade barrel
2. Better trigger
3. No more need to dry fire the gun to remove the slide during break down.
4. A choice of 3 different grip back straps to allow you to pick the one that makes the grip fit your hand the best.
5. Contour of the slide and slide serrations, which should improve grip for racking the slide.
Now specifically about 9mm. The XD magazine will hold 16 rounds. The XDm magazines hold 19 rounds. I'm not certain, but believe that they are the highest capacity you will find in a currently made pistol as a standard mag.
I loved my XD .40 and now that I have an XDm .40 I am even happier. I don't think you could go wrong with an XD or an XDm.
I have ax XDm 3.8 Compact (9mm) and absolutely LOVE it. FLAWLESS since the day I bought it. Also... it carries nicely in IWB (Maxtuck from White Hat Holsters). Friend of mine w/ SW M&P .40 EDC shot my XDm 1 day and raved about the XDm trigger as compared to his 40. He then let me shoot his M&P.... STILL LOVE MY XDm!!!!!!