What, if any is the main difference in capabilities between the WGE and the Client/Server engines?
There are no real differences in capabilities between the engines except for security and the operating systems that the engines run on. Workgroup engines run on either Windows 9x or Windows NT and as such are generally a bit less stable than the Client/server engines which are designed to run on a true server OS and take advantage of the security built into those operating systems and networking subsystems.