The chances are that the more someone knows about other OS's like Linux, the less they will know about Windows.
I'd choose the person with the most Windows server experience everytime, because that's where the problems arise that cost the most money to fix and can warrant the most money to fix. I've been in the IT industry for 13 years and only ever need the most basic knowledge of Linux, I've had to know how to do everything in Windows. DOS is actually more of a concearn than Linux for a lot of UK companies.
If you think I'm putting Linux down though then you got me wrong, my experience is that Windows systems require the most maintenance, and people who use Linux are usually more technically adept than the average Windows user, so less babysitting is needed.