Pros of Mac: No viruses, good looking, easy to navigate
Cons of Mac: Not all programs run on it, must buy specialist Mac Office and games, etc., only runs on Apples
Pros of Windows: Many things will run on it, easy to use
Cons of Windows: Many viruses, target for hackers
Which is better for the web developer (Adobe Creative Suite and Macromedia Studio): Windows, just because you will be developing for a target audience on Windows, so it's much easier to design things and then view them. Also, larger range of tools.
But Mac will only run on Apples at the moment, as they have a different instruction set. You will have to either hack and remodel the Mac OS, or wait until there are Intel Apples.
Stability and security won't be too much of an issue, stability maybe though as it's such a graphically advanced OS. Many people will turn to Linux and Mac OSes because of DRM, I think at least 5% will, that's my guess at least
The DRM stuff, like Windows being able to shut off VGA ports if it thinks your recording off them, and of checking your PC for illegal material, is dictatorship - Microsoft would make themselves dictators