I have both. Both run the same as far as I can tell (with the exception of the RAM). However, it will come down to what hardware you have and if you have any special software that may not work for whatever reason. Although, I would assume by now that most of the driver issues have been sorted out unless you have some serious legacy hardware.

Also, I built a new computer for it. So, I never actually upgraded. I had no issues that I recall.

I turned off Aero but not UAC.

edit: I just remembered what I thought when I first installed Vista 64...why the hell didn't I try Ubuntu with WINE first? Since my box is pretty much dedicated to WoW, I thought this may have been worth attempting and save me some money [and make Bill Gates $5billion+++ - $150 less rich :P (yes I know this isn't true, but it makes me feel better to think that way)]