AppleInsider | Shootout: Parallels outperforms VMware Fusion in many testsYikes! Rules out Vista for me. You really want XP inside these VMs.
... 3D and HD Graphics Tests
XP: Smoothly played both 720p and 1080p videos in both environments.
Vista: Couldn't play 720p at all in Parallels. VMware Fusion stuttered on every machine except the Mac Pro. Given 720p results, MacTech didn't bother trying 1080p...
Parallels was significantly (20% range) faster than Fusion, but for me speed is less critical than reliability and stability. They didn't seem to test that.