I thought he was asking for opinions about the XDm, not S&W or Glocks.

Also, there are a few differences between the XD and the XDm, those differences are all for the better.
I bought an XDm-9 a few months ago and absolutely love it. I dedicated a few hundred dollars to researching this purchase renting most popular models in both .40 and 9mm. For me nothing felt quite as good as the XDm and I was shooting a lot tighter groups with it as well. It is my first handgun so a lot of factors contributed to me getting the XDm-9 over the XDm-40; large magazine capacity (19+1 is amazing), 9mm rounds are a little cheaper than .40 rounds and most of my buddies have 9mms.
Unfortunately I didn't do a whole lot of research into the aftermarket or ballistics. Had I done that I would have bought the XDm-40 instead. The .40 S&W round has better stopping power than the 9mm, and there are conversion kits available for the XDm-40 to convert to both .357 sig and 9mm. That means for about $150 (plus extra mags) you could have three guns in one.
That being said, I still really love my XDm-9, but I would love to trade it for an XDm-40 if anyone is interested. It has only a few hundred rounds through it and is in "as new" condition.