Well, same memory support, but server can support up to 4 processors while pro does "only" two. Server comes with a program called Terminal Server that lets you run programs on the machine from over the internet or LAN. Cool if you have another computer, but its outdated and couldn't run things as fast. There are other cool things with server. Keep in mind that when you buy it it will come with 5 client liscenses (or more if you are rich and have money to burn) which means you have the right to install Pro on 5 other computers. In fact, pro comes on the cd also. Basically almost the same core with a few key file differences, as well as the management apps. Just checked pricewatch and you can pick it up if you are a student (academic bundle) for $445.
Basically unless you want to serve up some quality games or manage a network a lot easier and more securely I would just stick with 2k pro, especially if it is stable, which it usually is.