I see the ability to run "top" as a normal user on a KVM host and see what the guests are up to as a big advantage. Sure, one can run xentop on Xen, but only if you have root access.
If you have at least read only access to libvirt virt-top is nice to get more detail on guest stats...
Have you looked at KVM with a c6 host yet? It's a marked improvement over c5 hosts...