For some time, I have had both VMWare Fusion and Parallels Desktop for Mac on my MacBook. As part of my Mac "diet" to slim down the data on my hard drive, I decided that I would only have one virtualization program on the MacBook Air, so that meant that one had to go.
I have used both programs, and there are things that I like about each of them. They offer very simliar features and options across the board. However, I have used Parallels more frequently and for a longer period of time. Nonetheless, I wanted to give Fusion a fair opportunity before making my final decision.
I used the VMWare Importer Tool to pull a copy of my Parallels "virtual pc" into Fusion so that I could compare apples to apples (no pun intended). It did a pretty good job, after about 40 minutes, of pulling all of the programs, etc. into a Fusion virtual machine. I installed the "Tools" functionality in Fusion, and I then began my comparison.
Both virtual machines used approximately the same amount of disk space (21 GB). Both took approximately the same amount of time to start up and allow me to begin using the Windows programs, and the programs themselves seemed to run at a comparable speed on both virtual machines.
Unfortunately, the VMWare Importer did not bring my print settings into Fusion, which proved to be a huge problem. In fact, I tried for over an hour to configure the settlngs to be able use our network printer at the office, but in the end I gave up. To be fair to Fusion, I also tried to print from another Fusion virtual machine I had previously installed, and it wouldn't allow me to print either.
So in the end, I decided to stick with Parallels and not install Fusion on the MacBook Air. It should not be so difficult to set up printing in Fusion, and that ultimately proved to be the tie-breaker for me. I invite you to submit comments as to which virtualization option you prefer and why.