I use both for various things and depends what you want them for heres my quick opinion:

VMware Fusion: Faster, better support, speed improvements make it good for most things but lacks Directx9 support (it only does 8.1 and very bad OpenGL).

Parallels: Far more features DirectX9 Support and very stable for the basic stuff but sluggish and some features are kind of buggy or not very well thought out
eg "Bootcamp Support" which is great in theory until you have to content with windows activation which makes you reactivate windows whenever you go to bootcamp and have been running it in parallels or vice versa because of "hardware changes", until your windows activations expire and you have just wasted a full copy of windows.

Overall I wouldn't use either for gaming even with 4 gigs of ram and a core2 duo you still can feel that sluggishness in some ways.

General applications you can go either really its a matter of taste, if you need support vmwares the way to go, if you like all the bells and whistles parallels wins hands down.