This is all interesting. IMHO, the only reason Windows is exploited more is because it is hated more. Myself, I have a love/hate relationship with my Windows XP boxen and thinking a little more about it, I always have. If Windows did not make me money, it would be easy to dismiss it, but the fact remains that it is necessary because big business does not want (or can not afford) an infrastructure change on the magnitude it would take to drop Microsoft. No company wants to be the first to go against status quo :-)
I am dropping Windows on my home system because I am tired of focusing on vulns and viruses and having to spend money on products that I need to keep my computing environment safe. I have looked into Linux and I have owned and still own a Macintosh system... the one I currently have is an old 7600 running OS 8.6 (circa mid 1990s). I was a FreeBSD user for a long time and MacOS X is a very big advancement for Apple! That doesn't mean it is secure though.
Someone mentioned that BSD is just a kernel... MacOS X (not sure about the current Panther/Tiger) was all FreeBSD with a lot of new utils and a pretty GUI put on top... the utils are indeed what separates "Darwin" from FreeBSD, but you can run most of the same commands at the prompt and it is still FreeBSD under the hood. That having been said, the same exploits that may be in FreeBSD (not secured) will be present in OSX.
It USED to be that Windows was the only system to have viruses and Macintoshes and *NIX systems could be carriers but not infected (ie: send an email from your mac to your buddy with a PC and you could infect him if you carried a virus)... Spyware of course is a big problem, but given time, it will be a problem on other platforms too.
As MacOS X gains market, vulns will be found and exploited because everyone wants to unseat the king of the hill, again this is my opinion only :-)
My personal choice... Go with Amiga and specifically get to A1... that will be the platform to watch... and at least for now, I will be free of headaches outside of work.
an aside, The benefit of working for a company is they provide another win box for me to use anyhow, so it is safe to change my home platform.
Anyhow, all platforms have potential to be exploited.