I have a .40 caliber XD service and I have both an M3 and an x200.
My first light was the m3 due to price, it cost me right about $90 for the light, batts and an extra bulb. The x200 i recently acquired for $200 (I paid no tax or shipping) and it came with batts and mounting hardware for picatinny rails and a mount for 'universal' rails (your XD/glock/etc).
I've fired about 250 rounds of target load thru my shotgun with my M3 attached and about 100 rounds of UMC ammo thru my XD with the M3 attached. After all this shock and vibration it still functions like the day i purchased it.
I like the M3 a lot, the adjustable beam is nice and i have one of the pad-type switches that i use with my shotgun and stock rotary for my XD. My only concern about the M3 is it's water resistance as it is a plastic housing. I know it's not a huge deal when thinking of a home-defense situation, however, i carry mine when i hunt and sometimes this becomes an issue. The x200 is water resistant to 30 meters and features a sealed metal housing. i'm not entirely comfortable with the x200's momentary-on style of switch yet. it's a push-forward motion which is a bit odd to get used to at first as the M3's is a push up for momentary, down for constant. Both are about the same size, the x200 being slightly smaller, so any holster that will hold your XD with an M3 will hold your XD with an x200.
The beam from the x200 is at least 2.5x brighter than the M3 and both are rated for the same battery life, about 1 hour continuous. after the one hour for both, the lumens rating falls off and it's recommended that you change the batts.
I think that you'd be more than pleased with both, however, if you have the cash i'd shoot for the x200.
I'll try and dig up a digital camera so you can see them mounted, both fit very nicely on the service model.
best of luck with your decision,
-Chris
