Compared to the original XD The XDm has a slightly different grip shape and a funky checkering - grooving pattern that gives a pretty firm hold for bare plastic. The XDm does not require pulling the trigger to disassemble. The XDm magazine is NOT interchangeable with the XD of the same caliber except the .45.
Sorry Knifecollector, "match grade barrel" is pure advertising hype. It doesn't say "match WINNING grade barrel" after all.
The barrel is a bit longer on the XDm (4.5" vs 4") and the grip has interchangeble backstraps to better fit the size of your hand. I have the both the XD and XDm in .40 S&W. The ergonomics are better on the XDm but you will need to decide if it is worth the extra money.