It’s funny, I’ve been using a Mac for more than a decade now but I still think it comes down to personal preference. I actually quite enjoy Windows 7 and I’m hoping Windows 8 will close the gap between my enjoyment of both OSs.
While I do all of my work (web development) and a Mac, I do bootcamp into Windows 7 to play games. I think that’s the best part about owning a mac, the fact that you can run both operating systems is fantastic.
There *are* some apps that are only available on OS X, but I think they are more ‘nice-to-haves’ rather than ‘essentials-for-design’.