Given the way that Linux and Windows are licensed, using Linux would cut on the OS licensing costs, and Linux would make a better use of the CPU as you would only load the modules and process you need, with greater flexibility than Windows.
Also, you can run a server-only Linux, which is not possible under Windows, as well as running a headless computer, which you can access using a simple SSH connection. You may want to take a look at DistroWatch:
They will list all the major distros as well as those that are geared towards server configuration. The LAMP (Linux Apache MySQL PHP) configuration is what you are looking for.