I've got both a XD Service in .40smith and a XD Tactical in .45acp. The Tactical (5in) is nice, a little nose heavy (but that helps with muzzle flip), seems a bit more accurate (longer sight radius may be helping there). The Service (4in) is a nice pistol as well, no slouch on accuracy, and is fun to shoot.
I do, however, prefer the full length guide rod on the Tactical versions over the Service versions' captive spring, 2 piece type guide rod. With the .40smith, you'll also have the option of getting conversion barrels. You can change to .357sig with the swap of a barrel or can go to 9mm with the swap of the barrel and magazines.
Whichever version you choose, I'm sure it will serve you well. My preference is for the Tactical (5in) version though and I shoot the .45 a lot more than I do the .40!