Make sure you guys are talking about the same thing. Halogen bulbs are typically all the same size so a swap is relatively easy. On the xenon front, I've seen different length bulbs so that is something to be aware of. Clearance may be an issue when looking to replace your HID (xenon) bulb in the projector housing. If the HID bulbs are of different size that can have an impact on how the projector focuses the light. I'd certainly measure the OEM xenon length before ordering any aftermarket HID bulbs.
And just for the fun of it, here is a shot of a halogen next to a HID bulb:
http://x3.xbimmers.com/forums/showpo...97&postcount=7