I prefer Fusion as I'm a vmware admin and it feels natural to me.  Portability factor also counts as i can use vms that i create in Fusion to run on windows workstation, linux and ESX servers.  I'm biased towards VMware and never used Parallels.
So far version 2 of fusion has been very stable.  I frequently run 4-5 VMs pretty much every day to keep my skills upto date on other vmware products.  i'm not allowed to do so at work since those are production systems.
So far I have been able to install and successfully run all vmware products inside Fusion without any issues.  Only issue i run into is low RAM count as my VMs consume around 7GB out of 10GB that I have on my Mac Pro