Check out th What's new guide to know the features released vSphere 5.1
Intial release did have some issue which were released with vCenter 5.1.0a & b.
Check out release notes to know about the known issues
http://www.vmware.com/support/vsphere5/doc/vsphere-vcenter-server-510b-release-notes.html
It its new deployment then I would sugges to go with vSphere 5,1