Head-to-Head: Parallels Desktop for Mac vs. VMware Fusion
http://www.mactech.com/articles/mactech/Vol.25/25.04/VMBenchmarks/index.html

So which virtualization product do you choose? Which solution is faster? Should you run Windows XP or Vista? 32-bit or 64-bit? One virtual processor or more? In short, there are different answers for different people. It all depends on your needs.