I only have limited experience with VMWare's offering, but one thing it can do which MS's offerings cannot do is run 64-bit virtual machines. To do that though, you need to be running a 64-bit OS as well.

Microsoft's products can run on a 64-bit OS, but they are only capable of running 32-bit virtual machines.

Another possibly useful feature is that VMWare offers USB support on their products while VPC and Virtual Server do not. If you require USB support (dongle or something), that may make VMWare the more attractive option for you.