depend on who´s paying you
if you want to cinstall and configure a LAN for a phreaky model agency you should prefeer MS windows but if you want to make a site devoted to video games with forums, dedicated servers, email service, etc, the way to go is absolutely LINUX, it´s free, it makes you learn about computing, and of course it´s STABLE.
i´m not a linux guru but i´m sure open coude source it´s the future of programming (if Bill allows it and we agree).
but the MOST important thing about linux is SECURITY