Cost

For desktop or home use, Linux is very cheap or free, Windows is expensive.

For server use, Linux is very cheap compared to Windows.

Microsoft allows a single copy of Windows to be used on only one computer. Starting with Windows XP, they use software to enforce this rule (Windows Product Activation at first, later Genuine Windows). In contrast, once you have purchased Linux, you can run it on any number of computers for no additional charge.

I don't know if there will ever be an objective measure of the ongoing care and feeding costs for Linux vs. Windows. If there were however, it would have to consider:

Dealing with bugs in the operating system 
Dealing with bugs in application software 
Dealing with viruses, worms, Spyware, etc.(big advantage to Linux here) 
Dealing with software upgrades to new versions (both the OS and applications)