Since Vmware vSphere 6 is released the vCenter Virtual appliance (VCVA) has the same specifications as the Vmware vCenter installed on windows (VCS). So you can install the VC virtual appliance without to worry about limitations and also one windows server license less to buy. Downside is that there is nog migration path from VCS to VCVA, so before you upgrade to vSphere 6  you can first migrate to a VCVA 5.5 and the upgrade your virtual appliance to vSphere 6.  there is a cool tool do this from Vmware fling website "The VCS to VCVA Converter Appliance is the winning [...]