Wait.. A posting not about SharePoint? How can this be? Well, it's somewhat related to SharePoint, since so many people use Virtual PC to host a development SharePoint environment. Since SharePoint requires Windows Server 2003, that's the best choice for us laptop users. Anyway, this is just a quick comparison between some points of these two products.
Map a virtual NIC to a wireless physical connection?
Compact disk image to a separate location?
Management tools for multiple guest OS?
Runs as Windows Service(can auto-restart after host restart)
*Performance really can be very good, on two conditions: enough physical RAM on your host (2GB will do, but 4GB is much better), and putting the virtual hard drive on a physical drive other than the one holding the host OS. On a laptop, a USB 2.0 external drive works very well. Be sure to "safely remove..." the USB drive before unplugging it.
Bottom line: if your host is a laptop or desktop, use VPC. If your host is a server in the datacenter, use Virtual Server.
Sometimes people ask me about VMWare's products. Well, I don't run them so I can't say, but Andrew Connell has a good blog posting examining some of the differences, especially related to how we use them as SharePoint developers, consultants and trainers.