
There's not a lot in it really, I used to use Fusion because I'd tried both and I preferred it to Parallels. I saw a thread in here which mentioned that Parallels had come on a lot in V4 so I gave it a try again and I now prefer Parallels to Fusion. I've also played briefly with VirtualBox but I couldn't even get an OS installed using it!
Six of one and half a dozen of the other really. One becomes better than the other, then the other catches up, etc etc. I'd say that it's just down to which one you feel happier using.