Whenever two options are presented, it is often a smart move to weight the pros and cons of each and make a decision based on your needs. If you need to push Windows in a virtualization environment, Daniel A. Begun at CNET Labs has benchmarked Windows Vista running in both Parallels Desktop and VMware Fusion. The short of the long? If you really need to push Windows, do it in Boot Camp if at all possible. If Boot Camp is out of the question, VMware Fusion was the winner in heavy duty, multimedia tasks by a landslide, due largely in part to its support of multiple core processors (Parallels, as of this writing, only supports one core per CPU). On the flip side, however, Parallels Desktop is the only virtualization product that supports 3D gaming right now, but as a Parallels owner, I'll still vouch for running your games in Windows via Boot Camp.
At the end of the CNET Labs day, both apps still have their pros and cons. Begun hails Parallels Desktop for being the more usable and intuitive of the two (and during my initial tests of both before deciding to buy, I agreed), though I had a much easier time getting non-Windows OSes installed in VMware Fusion (to this day, Parallels Desktop still won't install Ubuntu, and I have received no response as to why). Still, the moral of this particular story is that if performance is at the very top of your virtualization shopping list, VMware Fusion is, at least for now, the reigning champion. Check Begun's full article for more details on the tests and how they were performed.