Zodder, speaking only from my personal experience, I havent had any problems with games in 2K nor XP.
Unlike what some people seem to think, 2K doesnt have a compability tool as well, only it isnt installed by default, it's on the CD.
It works very good, I've had a few games that have complained about not working on NT(Motorhead comes to mind), but the compability tool took care of them nicely.
But if you dont have either and will be buying new, just go XP; once you turn the crap off, its like 2K with a few enhancements.