Well. Good question. I suppose your choice should be based on being the cheapest/best for what you need. Your clients are probably on Windows, however a free Linux Distro like CentOS can certainly handle DHCP, printing, Files hosting, etc.
I guess it comes down to exactly what applications you need, and the type of database. If you need .net and sql server, you gotta go Windows. If you can hanlde MySQL, go linux.
As far as hardware, get the biggest fastest thing you can afford.
You didnt list it, but Sun is also a good Server option.
http://www.sun.com/servers/index.jsp
Edit - Point taken... you can just get the Solaris OS (I did), but you couldnt use it as a business server (legally). However, you can get cheaper Sun Fire servers running AMD Opteron (not SPARC) starting at around $1000:
http://www.sun.com/servers/index.jsp
But, if you are not planning on hosting Java web applications, Sun might not be the way to go.
You mentioned Linux developers, and basic Apache/PHP/MySQL hosting. Then you might aswell go Linux... what are they developing for? Red Hat? You could even go with the same Linux they are working on.