Ubuntu Server or Ubuntu Desktop?

Whether you choose to use Ubuntu Desktop or Server depends on whether you plan to use your box primarily for ZM or as a multi purpose box

The key differences between Ubuntu Server and Desktop are:

• The server version does not have a GUI installed (which means you can't really use solutions like TeamViewer or X11 based remote graphics systems until you install a graphical desktop)
• It has fewer processes loaded at startup
• It has lesser packages installed

Personally, I'd strongly recommend you go for Server instead of Desktop for the following reasons:

• It is faster
• Takes up less memory
• You can ssh into it any time and interact with it
• You can use byobu or other interfaces to use multiple terminals etc. (don't expect them to be as nice as a GUI desktop)
• If you really want later you can always install a desktop. If you are installing a desktop GUI, I'd recommend lubuntu

Note: Some folks recommend using a web based UI for ubuntu server called "minfy". Well, some screens of minfy need Java, which I really don't like. Plus I use OS X, and Java is always a pain there. I use byobu.