Now that Apple has gone to Intel hardware, the main thing that defines Macs is Mac OS X. The debate really is Mac OS X vs. Windows XP and Vista. I know some people prefer Windows and I don't have any problem with that, there are a couple of things I think are cool about Windows (resizing windows from every corner, great games), but these days it's just such a dangerous OS to run for mission critical work. MS is trying hard to beef up security but I don't know how much more can really be done.
A lot of what motivates hackers is politics... and as long as Microsoft represents greed, corruption, and incompetence, hackers will try their hardest to keep Microsoft busy writing security patches. Sucks for them, reading some of Microsoft's official developer literature has given me some sympathy for the issues they face, but it doesn't make me want to use their OS for serious work.