Hardware-wise, I like Windows over OSX because I like to build and upgrade my own PC's. I also avoid the cheapest pre-built Windows PC's because they put the cheapest components inside and bloat it up with useless software that has to be removed. I prefer to do it myself and choose components of decent quality. Once you hit mid-range performance, it ends up being cheaper to do it yourself anyways.
Software-wise: From a usability standpoint, the OSX interface is foreign when I try it, but I think if I used it more it'd be just as good. Lack of software is a problem, and buying a copy of Windows on top of OSX is more money. Viruses are a bigger threat on Windows, but the most important anti-virus sits between the keyboard and the chair. And like many people said, Macs are not immune. Here's the latest on Mac malware: