I just signed up to WoW and have the exact same situation as you. I find it much more convenient on the Mac side, and seing as it's going to be the same graphics card, there are no graphical differences. I do use the Windows partition for WoW thought because I have dedicated that HDD space for gaming (WoW install is about 500MB - 1GB). You can still talk and play with users on the opposite OS though, whatever you use.