Software/web developer here... honestly, the answer is, it depends.
My wife, a not-very-techie, who is mostly interested in sifting through pics and playing on myspace, chatting, etc, after an initial break-in period, now likes vista much better (photo gallery features, etc). On an "older" system, for performance reasons, I'd tend towards XP.
I personally find a good general rule with all of the Microsoft OSes, after SP1, each new OS is usually preferable to the previous. I would have a hard time making an argument against buying a PC with vista pre-installed, though I'd be interested to hear why some people might feel that way.
Also include me in the camp for Windows 7. Beats either of the above very easily.