Linux and Mac are based on the UNIX theory, the theory that has passed tests and serving for at least four decades. so they are for sure great and robust systems. Windows is based on, DOS? Sorry, but I must say that windows is based on some thing that has given to the world more headaches, shame in public and a lot of stress (due to system hangups, BSODs, security vulnerabilities...). As a robust system, I prefer Linux.
Windows has something that any Linux software is still far away from: an easy to learn, consistent and user friendly user interface. But let's see mac. Although they didn't invented the GUI, they have been mastering it for two and a half decades. It doubles the time of a nice windows ui, and its about three times of the first low-end user linux gui. Mac have been the leader and the model to copy when talking about UI. They are the creators and masters in the current glossy look trend, that is being used in Vista (take a look to a
video on nytimes, and this
article on lifehacker ) and in important linux projects like
oxygen and crystal clear. As a system with a beauty and clear UI (that improves productivity), i prefer mac.
But what if you need for tomorrow before sunset an application that gets data from a web service, process it, send a mail with results and show all the results in a desktop application to your boss/father/teacher/whoever?. Hmm, should I do it with QT or GTK libraries? What libraries should I use and what dependences will have my app?. Or should I do it in Objective C?. Well, i definitely would choose the .net framework. Yes, thats windows. Developing applications (web and desktop) with the .net frameworks goes straightforward, in a simple and organized way. And I know that Visual Studio isn't the cheapest tool, but I prefer to open one real IDE, than trying to use and integrate lots of applications that make one task. As a platform which is easy to develop for, I prefer Windows.
I agree: the question is what suits you.