Roy,

here is a link to a paper that explains the key differences between VMware vSphere and Citrix XenServer: http://www.vmware.com/files/pdf/vmware_advantage.pdf

As you'll see, there is a substantial difference between the two and vSphere is far more capable and complete solution. I don't know what your experience is with each product solutions, but once you put the two products to test in real life, you will find a great difference not just in terms of features, but also of ease of use, config, managebility, support, etc. 

Can you provide more details around your impression? I may be able to provide more specific answers.

Alberto