MacTech Magazine has published a set of 2500 benchmark tests that conclude that Parallels Desktop 4 runs Windows on Mac computers faster than does VMware Fusion 2 in most cases:

In the majority of overall averages of our tests, Parallels Desktop is the clear winner running 14-20% faster than VMware Fusion. The one exception is for those that need to run Windows XP, 32-bit on 2 virtual processors, VMware Fusion runs about 10% faster than Parallels Desktop.

MacTech reported that Parallels Desktop 4 was faster in launching Windows, CPU performance, file and network I/O, and in running Internet Explorer, according to MacTech. The only test suite where VMware Fusion 2 performed faster was in launching Windows applications. Game performance varied among different games, though Parallels did better on the low-end MacBook. MacTech also said that Parallels Desktop's footprint on the Mac was smaller than VMware Fusions, particular in the use of RAM. MacTech describes some of its conclusions regarding different operating systems:

For 32-bit Windows OSes, running under a single virtual processor (the default when you create virtual machines in either product, and therefore, the most commonly used configuration), Parallels Desktop runs both XP and Vista 14% faster than VMware Fusion...

For 32-bit Windows OSes, running under two virtual processors, VMware Fusion runs XP 10% faster than Parallels Desktop, and Parallels Desktop runs Vista 20% faster than VMware Fusion...

For 64-bit Windows Vista, running under two virtual processors, Parallels Desktop runs 15% faster than VMware Fusion...

The article also describes the methodology used and includes a link for a download of a spreadsheet containing all results. Comment below