Hello gk26
In my experience, I like to use the SQL Server standard when the environment is for production, even if mission critical or not. The reason is that the business is running and as a IT responsible we need to provide a good solution for the bussiness.
Even if, you need for a Dev o labs, an sql express could provide you a good experience for 50 vms...
here you have a topic about this for vcenter. http://communities.vmware.com/message/1613422
Diego Quintana