I have two set-ups...FWIW:
1) Streamlight TLR-2S on my XD .45 Tactical
2) Insight X2L on my XD9 SC
Happy with both, but for different reasons. The TLR-2 is nice and bright, but large. That's OK on my home defense pistol. Dual CR-123 batts and C4 led really light up a room.
The X2L is a great little light that lends itself to concealed carry. Small single CR-2 battery does not get as bright as the much larger Streamlight.
Your choice of light will depend on how you plan on carrying your XDm40.
With holsters, I am looking into BladeTech for both of my XDs. They have a larger selection of belt/molle/drop-leg fullsize holsters for the .45 Tac and TLR-2S. I am looking into the WRS for the bigger gun. RH WRS Weapon & Tactical Light Holster w/SRB Loop :: Holsters w/Tactical Light :: Blade-Tech Industries