embedded 5.5 vCenter - vPostgres (prior versions used SQL Express) - 5 Hosts and 50 VMs
embedded 6.0 vCenter - vPostgres - 20 Hosts and 200 VMs
external vCenter- Microsoft SQL - 1,000 Hosts and 10,000 VMs
vCenter Server Appliance
embedded 5.5 vCSA - vPostgres - 100 Hosts and 3,000 VMs
embedded 6.0 vCSA - vPostgres - 1,000 Hosts and 10,000 VMs
external vCenter - ORACLE - 1,000 Hosts and 10,000 VMs (primarily for bringing in an existing database with content)
So... the vCSA now allows full size support and now supports Linked Mode, plus you avoid paying Windows License or MS SQL Licenses and the whole Windows Security attack vector.
If you are using the VCS and you would like to move to the VCSA you have two options:
Rebuild intelligently - really your best option
Use this fling: https://labs.vmware.com/flings/vcs-to-vcva-converter
So get to the VCSA - it's awesome!