CPU is only part of the equasion... and core duo is fairly new at that.
As for which is better, the only reason I use XP is cos I have to - M$ have marketed their PC like a hoe and everyone has just followed. Of course PC open architechture added to it, but admit it - OSX just looks better. Just google for the number of Windozw tools to make your PC look like OSX...
OSX= Cinderella
Windoze= Phunderella
Agreed that the CPU is only part of the equasion, but Apples and PC's run virtually identical hard ware in almost eveyway, in fact it's the same CPU's same RAM, same graphics cards, same hard disks and optical devices... infact the only component that really differes greatly is the motherboard chipset... even the CPU:Memory interfaces etc. are the same with the new systems.
Trust me, I am no big fan of Windows myself, in fact it's pathetically awkward and can be most annoying, especially the whole applications stealing focus thing.
But at the same time, OSX may look nice, but I find it close to impossible to actually attain any level of productivity on it... unless I open terminal
I have dabbled with Vista... it's much better than XP, basically making windows look almost as visually apealing as OSX, but at the end of the day MS is starting to make the same mistakes as Apple by making the interface too user friendly, which kills any productivity, as it's so much more difficult to figure out what has gone wrong when it inevitively does.
In Vista the dialup internet manager is so pathetic, it doesn't even tell you why the internet connection could not be established [I like knowing if I typed the password in wrong, or the remote computer did not respond or whatever].
