Well, as you already mentioned, the lack of an underlying OS (I suppose Windows in your case?) makes the whole thing significantly more stable and easier to manage and maintain in terms of patching and restarting the OS every month or other standard applications you use on your windows servers.
You just install ESXi, configure some minor settings, start up your VI Client and you are ready to go without having to worry about much other stuff. Just look out for important updates that get released every now and then. ESXi is just the way-more-awesome system and the performance should be significantly better on ESXi.
There is a bunch of other advantages (and also minor disadvantages in certain cases), but I'm too lazy to enumerate them right now.
Here are some comparisons between the products (unfortunately old and mostly server 1.0, but many points should sill apply for 2.0):
http://www.petri.co.il/virtual_vmware_versions_compared.htm
http://vmzare.wordpress.com/2007/04/02/difference-between-vmware-server-esx/
http://www.virtualization.info/2006/12/choosing-between-vmware-server-and-esx.html
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