You've got 4 choices:
9mm: Cheap plentiful, high capacity (15)
357 SIG: Neato wonder cartridge, not easy to come by. Holds 12.
.40 S&W: Practically de-facto police cartridge. Holds 12.
.45 GAP: Ballistically the same as .45 ACP, XD only holds 9 rounds. Have yet to see one at the local gun dealer.
If you just want something for target practice get the 9mm. That's the easiest answer there is. 9mm is the lightest recoiling, there is an obvious reason for that.
Me personally, if I got another XD I'd get the 357 SIG. That's just because I like the idea of a pistol shooting bullets at obscene velocities. You can't really go wrong with any of the XD's though, they are all great pistols.