I currently own Parallels and VMWare Fusion and out of the two I'd definately recommend VMWare Fusion over Parallels.
I find Parallels sucks down too many resources especially when unloading and quitting the app, VMWare Fusion tends to release memory and resources better after running Windows XP from both.
Seems strange since I have 6gb Ram on my Mac Pro QuadCore it shouldnt have that issue.
I also noticed Parallels would jam up when I quit it and then needed to load it back up at some point again later, the icon would sit in the dock doing nothing and wouldnt force quit.
VMWare have been out and at it for a lot longer than Parallels have too from what I heard.
