So is the traditional vSphere Client being phased out?
Yes that is correct, Starting vSphere 5.1 all new features are avilable only in vSphere Web Client 5.1, Going forward vSphere Client will not be avilable and only web Client would be source manage the infrastructure
A Comparison chatcan be found here http://blogs.vmware.com/vsphere/2012/11/which-vsphere-client-should-i-use-and-when.html