XP or Vista?

So I purchased a copy of Parallels Desktop a few months back, when they were offering it at a reduced price while still in beta. I haven’t gotten around to installing it since, mostly because I didn’t have a legit copy of Windows to go with it, and I’m not much interested in dinking around with any Linux variants.
Lately, I’ve been intrigued at the prospect of running Windows from a virtual environment on my Intel iMac, mostly for web browser testing. (My sites don’t look nearly as nice in Internet Explorer as they do in, well, pretty much every other browser.) And long ago I promised I’d help out with some of our church’s web stuff, and they use FrontPage (yes, I know–ick!).
The question then is, do I get the latest version of Windows XP, or do I jump in to the exploratory waters of Windows Vista? Let me know what you think.