I have both
I have both and generally lead toward Fusion due to its better USB, Linux support and the fact that it can run VM's from work with no conversion or hassles. I like Fusion's ability to assign more than 1.5GB of RAM to a VM and its dual CPU support.
But, the 1.1 leopard update seems to have made fusion a lot slower than the 1.0 release.
Parallels is a lot faster for me, but it is less stable and really is aimed at running just Windows.
Both Fusion and Parallels seems to burn a LOT of cpu even when idle compared to Vmware workstation running on my Linux box or my windows system. Idle, Parallels is 30-50% CPU, and Fusion is 25-37%. In comparison, VMware workstation idles at 8% on my Linux system.