XenServer and the xen.org stuff are very similar from the perspective of a guest. I mean, administering them is a little different, but not that different. there is rather a lot of shared code between the two systems; Xen.org generally gets newer features first, and xenserver has for-pay support.

Recently (well, I guess not that recently) the 'xl' command line from xenserver was given back to xen.org. it's a lot nicer than 'xm' I think.