Ironically, it's one of the biggest decisions you make when you get a Mac: How should I run Windows on it? Parallels or Fusion? An exhaustive battery of benchmarks by MacTech reveals a clear winner.
The short story is that in most cases, Parallels runs a solid 14-20 percent faster than Fusion, except in the rather limited scenario of running Windows XP 32-bit on two virtual processors.
Overall, running 32-bit Windows OSes with a single virtual processor, Parallels is 14 percent faster; with two virtual processors, Parallels is 20 percent faster with Vista, while Fusion is 10 percent faster with XP; and for 64-bit Vista, Parallels is 15 percent speedier. Depending on the task, the numbers vary—like transcoding MP3s can be up to 30 percent faster on Parallels.
MacTech's tests are ridiculously comprehensive, spanning multiple machines with tons of different applications—the whole them took a couple months—so if you want the full, chart-heavy breakdown, head over there: [MacTech]