My vote goes to the XD.40SC, absolutely love this pistol. Its my primary carry piece!
OK if it has to be a 9mm then the XD9SC gets my vote.
Are limiting yourself to the XD/XDm line due to pricing?
If not I would say get a the Springfield EMP, a micro 1911 it comes in .40 or 9mm. Springfield Armory
Also I would take it to a gun smith/engraver and have a reference or something to remind you why you bought it, and what you went thru to get the money to purchase this pistol. Make it something that you'd want to keep forever. A constant reminder of your service and sacrifice for your country ( like your tour date) or something that was positive about your service. That is something I would do, but thats just me.