I'm on the Mac side of things. Like a few have said, you'll never go wrong that way.
I do hear Windows 8 is a more useable OS than past Windows versions. But, there are compatibility issues with old software versions, etc. (Among other complaints I've heard.)
It's also not much like past versions of Windows... which would of course be a good thing.
Also keep in mind, you're buying a hardware set with a Mac. Windows is just an OS. So it'll largely depend on who makes the computer itself. I personally prefer the dependability and consistency of Mac's hardware sets for my business. (I run 4-6 machines at any given time.) A few hundred extra bucks for a top flight hardware set is nothing in the long-run when you're running business critical functions. So, even if you're going to go with a Windows box... do some research and get a good hardware set. Last thing you want is to be on customer support with some ****-can box-maker.