IMO, it really depends on if you're going to be working on the server itself. If you are, GUI can make things a lot easier, especially if you prefer dragging & dropping & multiple windows/desktops over commands.
For me personally, I run a server with no keyboard, mouse, or monitor (except when I first installed the OS). I then do everything remotely through FTP, Samba (file sharing), SSH, and web-based utilities like Webmin. This allows me to access what I need to even if I'm not at the desktop (laptop or remotely via the Internet for example). This also makes it easier if you start expanding your horizons & bring other people in to help with the server, because you don't have to do all of that down the line. And like sesmar said, it frees up quite a few resources that can be saved for your clients.
On a side note, I'm a little more partial to Debian.
Aptitude ftw!