I installed both Parallels 5 and VMware Fusion 3.1.1 on my Macbook. Here are some observations:

In Fusion’s favor:

• Fusion is more robust. I’ve got an ISO of a TrueImage 11 boot CD. Parallels just couldn’t boot it. Fusion could. I tried lots of different OS settings and “Safe Mode” and Parallels kept reporting VM crashed. No problems with Fusion. Ultimately, I created a VM with Fusion, and restored the disk image, and then I imported the VM into Parallels.
• Parallels can’t boot one of my Boot Camp drives. I’ve got a Boot Camp which boots Windows from a fully-encrypted system partition. Parallels can’t boot it. Fusion can, with no problem.
• Parallels messed up one of my Boot Camp drives. I’ve got (another) Boot Camp partition with a “plain vanilla” XP on it. At some point, I got a kernel panic in OS X, and my Boot Camp partition was left without a partition table.
• It appears that Parallels doesn’t really give a raw disk to a VM. When you tell it to use a raw disk, it can’t deal with it unless it is a format which OS X understands. e.g. I can tell it to use a raw FAT or a raw NTFS volume, and it mounts it in OS X and then makes the mounted volume available to the VM as a disk, but if I give it a raw disk which my VM can understand but OS X can’t, the volume is not mounted in OS X and it is not available to the VM.

In Parallel’s favor:

• It runs my plain vanilla Windows XP VM faster than Fusion.