I've used Win2k and WinXP for years on PCs I built myself. Also great machines with a great OS.
I prefer the OS X interface. It looks nice and it's easy to use. The default XP interface is Fisher Price and looks like ass. I turn it off because I prefer the utilitarian look to bad design.
Windows is snappier and quicker to respond to input. Also seems to run better with less RAM. OS X is a memory and resource hog, and even on a Power Mac it responds slower than I'd like.
Both OS X and XP are patched with similar frequency. Those who say OS X is inherently more stable or less prone to bugs are drinking the Kool-Aid. I've had OS X lock up several times since September. It's not bulletproof.
I see little to no difference when running resource-intensive apps like Photoshop and Dreamweaver.
As a reference, I'm comparing my dual 2.5Ghz Power Mac with Geforce 6800 Ultra and 3.5 gigs of RAM running Tiger with a P4 2.6 with Radeon 9600XT and 1.5 gigs of RAM running XP and Win2k.
Personally, I prefer OS X for day-to-day use and I'll probably stick with Mac when I buy my next machine because the Intel Power Macs will be out by then. I'll dual boot OS X and XP, maybe even Vista. Best of both worlds.
Use what you like. The real-world differences come down to taste/preference and nothing more, unless you're a gamer, in which case I'd definitely recommend XP.