Esxi performance cost money. I would say that 0 dollars xenserver will be much faster.
Are we talking about money or performance?
I've tested both with redhat support and vmware support.
Both had same performance.
Vmware has good points and xen others.
Vmware is much more compatible then xen with those specific clients kernel versions requirements.
Btw. I do not like flames
Congratulations on you xen virtual server.