Installing the server or the workstation involves nearly the same steps and effort. And AFAIK I know an image create for one should work in the other unless versions are highly incompatible.
Server is the new entry and its Free :) How ever I think its Free as in beer not Free as in Speech :(  Need to spend some time to figure out the license.

The future might be the Server , but for the moment I thought to stick with the workstation.

Read the following links for comprehensive coverage for you to come to a decision.

http://www.vmware.com/products/server/faqs.html

http://kontrawize.blogs.com/kontrawize/2006/03/vmware_server_v.html