<p>Which OS do you prefer to program on? Client or Server</p>

<p>There is a school of though that if you are doing (mostly) web programming (or other server based code), you should use a server OS for your dev machine, since that's closer to the environment where your app will be running.</p>

<p>Update: I should add, this is really directed to the Windows crowd </p>

Regardless of the operating system you're actually talking about, it shouldn't matter. Most applications you might write won't need to worry about the differences (if there indeed are any). Only in rare cases might you use some specific functionality that might only be available on a "server" edition of your OS.
There are other considerations, for example Windows server editions are tuned by default to give less priority and attention to desktop programs, and more attention to things like the file cache. Personally, I would always choose a "client" edition of my chosen OS.