Personally, I like both of them. They both have their strengths. Virtual PC 2004 is insanely easy to set up and get a VM running. VMware Workstation lets you customize just about anything. They both are a little bit better at running certain OSes, so use whichever one runs your guest OS better. I use them both on a regular basis.
I assume the VMWare server is similar in set up and customizability (is that a word? lol) to VMWare Workstation. Virtual Server 2005 is, like Virtual PC, not so hard to use, either.
Moral of the story: They're both free. Use them both!