Depends what you are doing. VMware is more powerfull and faster, but some people prefer Virtual PC for ease of use. For my needs VMware is superior as it has good support for non Windows based Operating systems as well as Windows, which is a plus for me as i like to test and muck about wih Linux.
There is a 'Shoot out' article comparing the two
hereVirtual PC comes free if you get that MSDN alliance thing, where as VMware Workstation edition is somewhat costly. However, here is a VWware server edition that does everything you need that is free to download, and if you already have virtual machines set up (unlikely, but you can download them), then VMware Player will do the job.
Both VMware Workstaion and Virtual PC have free trial peroids AFAIK.
I've been toying with the idea od using some old 8-10 GB hard drives i have lying around as boot drives for various OSs, but the effort of swapping them all around would be a pain in the arse, i guess a hot-swap HD bay is in order, but at the moment i'm not sure if its worth the hassle (or cost).