The answer is something of a moving target. Both products are new and re-designed to work well with Lion. They tend to be upgraded frequently and in tandem. In past versions, Parallels has been faster than Fusion by a considerable margin, but VMWare has had better support. Reviews are still not fully available but the initial input on Parallels is favorable. I have used both on my Mac Pro without issue, but I have found Parallels to be faster by a significant amount. I am now using Parallels 7 under Lion, and it is performing well without any issues. I have not yet upgraded Fusion to version 4 and, based on my initial experiences, I may not bother.