If you want a virtualization product (that allows you to run Windows alongside Mac OS X), and you want the best performance for the types of things that we tested, then clearly you need to run XP and not Vista. Furthermore, in our tests, both VMware Fusion and Parallels performed well, and were a good user experience. That said, Parallels was somewhat faster in general than VMware Fusion for XP.
If you want the best virtualization performance for Vista, then VMware Fusion is your choice. And, if you want to keep your Mac OS X and Windows environments completely separate, VMware Fusion’s design may be your better choice. (And, although we didn’t test it, we would expect VMware Fusion to have better multi-processor support if you really have an application that is designed to take advantage of it.) If your goal is tight integration between one or more Windows applications and Mac OS X, Parallels is the clear winner when running either XP or Vista. And, as we said before, if you want the best XP performance with the types of applications tested here, Parallels is not only faster than VMware Fusion, but it’s faster than Boot Camp on average for the applications that we tested.