Windows is too bloated (alas, I have to keep using it) and it's a pain in the ass to keep it stable after a few years.
OSX runs atop a *nix core. Sorry Apple, you've done an amazing job, and made things really pretty, but I prefer the original stuff.
oh, and NO DRM SHIT! HAHAHA!
As far as CAD goes, and architecture in general, you make if functional first, then you make it pretty. Just like programming, mechanical design, basically everything that isn't... well, art. You can be artistic in architecture, but it's by no means a requirement.
Most CAD software runs on windows, but could probably be easily ported to *nix or Mac as the good shit runs OpenGL accelerated graphics, not DirectX (poor Autodesk... they fucked up because they thought Vista wouldn't support OpenGL and switched to DirectX). Although, I did hear that CAD apps are running up to 70% slower on Vista, so that might be changing.