Fusion and Parallels both are a bit faster than VirtualBox and have more features.

Also, VMWare VMs will work with any VMWare product so if you are working with a team that uses VMWare elsewhere its nice to have.

If you are just trying to run a Windows app or two you are probably better off with VirtualBox since its free.

Because of freezes and VM corruption. I've been using Fusion for years and never had any problems. It's worth the $40 (look around for a $10 off coupon code), if you're using the VMs for anything more than hobby/testing random OSes.

I prefer VMware's method of handling dual screen guests better. VirtualBox creates two windows that each have to be manually full screened, Fusion just magically handles both monitors from the same window when you tell it to use both in full screen.

Fusion is much more "Mac-like" and feels at home on OSX. Virtual Box is great if you can't afford something else (and often sufficient), but Virtual Box on OSX feels too much like Virtual Box on Windows or Linux, it just feels slightly out of place.

I've had VirtualBox freeze on me a few times in just the last month or so I've tried it, but can't remember any Fusion lock-ups in almost 2 years of use.