Do you have, or do you plan within near future to get a structured internal IT setup? I mean shared account management (Active Directory alike), centralized security updating of clients, file and print servers, software installation push, and perhaps central backup of client PCs.
If so, then having multiple operating systems is a horrible pain in the ... neck. And this is true for any combination of Windows, Linux and OS X. In short, as a small shop, don't try to do this. There are ways of handling this, f.x. Active Directory Group Policies on the Windows PCs and Puppet for the Mac & Linux boxes, but it is very time consuming.
If all your computers are self-managed islands unto themselves, i.e. each employee keeps his own PC in order and shares data with the others over platform-independent webapps, then it doesn't make any difference IMHO. In that case, I'd say give the Windows guy the PC that makes him happy and productive.
Of course, you may have nothing centralized yet, but hoping to get there in the future. If so, then I'd say take a hard look at when you'll realistically be ready to do this.