OS X client already comes with Apache so you don't have to install it. But from what I can tell, the server version is just 10.1 with some administration apps, more netinfo tools, PHP and some other Apache mod's installed, and WebObjects.
There might be more to it, but that is all I can see that is diffrent. I think it also comes with a higher level of support from Apple (atleast I would hope so, since its suppost to be closer to Enterprise class).
iMac 800 15", Mac OS X 10.3.2
Dell OptiPlex GX200, RedHat Linux (ugg) 9