Definitely take a good look at the stream light series, The TLR-2 and the TLR-4.
Tactical Gun Mount Series - TLR-2® with Laser Sight | Streamlight
The TLR-2 runs on 2 CR123's with a pretty good run time for the light of 2 and a half hours at 160 lumens (available in a strobe model), and laser only for 45 hours.
With a 5 inch XD tactical barrel the light will be flush with the muzzle when installed
The TLR-4 is smaller runs on a single CR2 battery, and runs 110 lumens for 90 minutes.
With a XD Service/Tactical the light will be behind the muzzle. and as a bonus the TLR-4 will fit a XD Sub
If it were up to me i'd have to choose the TLR-2 for its aluminum construction and battery life.
Good luck with your Light/Laser Combo choice.