Version 3.1 of fusion is very good and improved things greatly over 3. I've used both Parallels 5 and Fusion and have stuck with fusion, it does everything I need it to and for me was more stable and consistent. They both do very similar things and to be honest if your only looking to run some windows apps and to integrate them into OSX then they both do that...so get which ever you can cheaper
A lot of the reviews reckon the new version 6 of Parallels whips the pants right off Fusion. Just what I've read as I have no experience of either but from what I have read I think Parallels will be getting my money. Either that or I will make Windows 7 my main OS!!!!!!!!!!!