Finally had time to install Xen on a couple hard drives. (Thermaltakes are useful!) Installed Xen's XCP v0.5 and Citrix's XenServer 5.60. Initial impression is that the difference between the two is in the user interface. Both installed what appears to be the same server and server console, though numbers are slightly different (Xen XCP = 0.5, Citrix XenServer= 5.6.0).
For now, there's no rush for me to experiment heavily with these. I have to take the main server down to play with these and current house guests get cranky if the "Internet is down" during the evening hours. It probably won't become a crisis until VMware removes para-virtualization from ESXi 4.x (supposedly soon).
Xen XCP looks like it's ready for someone to write a new interface. The current one involves Java and I learned to dislike Java control GUIs back when Gauntlet 4.0 was cutting edge. Nothing's simple and there's too much stuff missing. Haven't had a chance to install the management software for Citrix, so I can't yet comment on that.
I'll keep notes in the wiki as I go along.