I had a service call last week where the client was running a "client-server" environment, but the server was actually running Windows Vista.
Well... Put it this way, this situation simply didn't cut it.
I learned that I'm not the first Tech Support consultant that has recommended that he use Server software for this machine.
So how do you choose the right server environment?
Many people would quickly choose Windows Server, just because they aren't aware of other options... and some others would immediately get into a comparison of Linux vs Windows. However, the first thing you should always do is take an inventory of the applications that are crucial to business success.
This could be a long list, but the bottom line is that the right Server Solution is the intersection of all these crucial business applications. I agree that there is always a solution, and you can make a choice and figure out how to make it work later, but can your business suffer downtime?
The next thing I would suggest is where you are going to get your support. Chances are you will not be administering this server yourself. Find out what your IT Consultant and Tech Support team say. Find out where their strength lies and play to it.