I've tried both and actually slightly prefer Parallels. Fusion is slightly quicker at the moment, because it supports dual cores, but the very latest Parallels (updated 2 days ago) and narrowed the gap.
I've been using Parallels for almost a year and never had any issues with it. The ability to launch OS X apps within Windows or vice versa is pretty funky (Fusion can do this too, but I prefer Parallels' implementation).
Fusion seems to have better Linux support for Ubuntu - but I run SuSE anyway.
Shop for your Apple, Mac, iPhone and other computer products on Amazon.
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.