The answer to this depends very much on the nature of the tests you need to execute, and the applications you are developing.
NCrunch itself can run on either O/S without any real difference.
If you are developing desktop applications, I would definitely recommend Windows 8.1, as this would mean testing your applications in an environment more closely aligned with your users.
If you are developing server-side applications (i.e. ASP.NET), Windows Server 2012 is probably the best way to go.