Here's an honest review:
I've used win2k, and experimented with 2k3, and I used them for a long time.
I recently installed Gentoo Linux, and let me tell you, it is the best thing i've ever done with my servers. It has some great feautres, and you learn so much installing it.
Windows is great for small sites, that don't get but maybe 50 hits a day -- if that. And keeping nothing really important on the site (for obvious reasons).
Again: as they have said, do what you know best. If you have some spare time, learn BSD or Gentoo
You will thank yourself later :P
Jacob