From my superficial experience with OSX Server, it's like Windows Server in the sense that it is OSX Plus. All the normal stuff is there, then server specific stuff is there. That includes both nice GUIs on top of open source stuff (Samba, etc), and nice GUIs on top of totally custom stuff (Calendar sharing, Wikis, etc).

If you have normal web-serving, open source using needs, regular OSX will be just fine. Be sure you really need server before buying, regular OSX can do any server type things that linux can.