LOL Who the hell brought Mac's into this discussion, leave them out as OS X is a completely different OS to XP and Vista. I've seen this argument so many times on so many forums that I can't even be bothered to have an opinion on the matter.
I'm building my new PC this coming week though, so I actually have the decision of what OS to put on it - XP or Vista.
Now I actually thought about it, and taking into account certain incompatibilities faced by Vista, opted for XP. However, in the future I may well want to take advantage of Vista's superior media features, so what I've actually done is acquired Windows XP MCE 2005, and will be using that now. Exactly the same as XP but with the added media center you see in Vista.
Basically a win-win situation, for my needs anyway.