Longhorn is called vista now, not vise versa. Longhorn was a working project name or codename in the early development stages (the first closed Beta I played with still said longhorn, I think).
If you have a new system using a directX 10 video card vista can take advantages of features that xp can't, if you have an older system or cheaper card that's still using DX9 or lower, you'll probably benefit more from the lower overhead of XP. Most programs (including games) will work with vista, as well as most of the critical hardware (network, audio, video, chipset, ect,...) drivers but very few pre-vista peripherals (printers, webcams, tuner cards, ect,...made before vista was released to the public) have drivers.
If you have particular software or hardware that you NEED to have look into it's compatibility on a case-to-case basis and keep a dual-boot with XP if you to have your cake and eat it too.