1 Unix-based server = 4 Windows servers

Microsoft always chooses the best for its web ventures and that, according to ex-Microsoft employees is Unix (Solaris, FreeBSD, etc.):

For the most part, according to our ex-Microsoftie, the company's money-making Web properties are all based around Unix, with Hotmail 99 being 99 percent FreeBSD, MSN using some Apache on Solaris, bCentral ad servers on 100 percent FreeBSD, and WebTV pretty much entirely Solaris.

And in cases where Microsoft does use its own software for servers?

... Microsoft had to quadruple the number of servers when it moved to its own operating systems.

All this says a lot about Linux. Although Linux isn't based on actual Unix code, it has been designed to be a virtual clone of Unix. And there are lots of Linux servers - about 35-40% of the Internet servers are Linux-based.