I have used both Mac OS X (and several Classic Mac operating systems) and Windows. At work we have special software for Unix computers. I have extremely limited experience with Linux.
Anymore I don't care who built an operating system-I just want to be able to do my work. Frankly I like Mac OS X quite well and I really have no problems with Windows XP. The problems I have had with Mac OS X are compatibility with scanners in particular and also photo printers. I started to get fed up when I had to wait forever for software to get upgraded so that I could do borderless printing. And it can drive you insane trying to find scanner software that will work properly. Bur for security the Mac OS cannot be beat in my opinion. All unnecessary services ar turned off by default, there is a firewall that you can turn on, and the Root Account (Superuser Account) is disabled by default. It is extremely easy to turn on and turn off various services. So easy Windows XP is put to shame in comparison. But Windows XP can also be run in a limited account to help protect the computer.
I don't know if there will ever be a day when the Mac OS X is faced with as many threats as Windows. But it IS possible for evil people to develop ways of attacking Mac OS X. Presumably if Mac OS X were more commonplace there would be more threats developed. I am not a programmer but the above to me seems logical.
Here is one thing to consider, however. If every Windows user would just buy and use and update an anti-virus progam, use a firewall, use an anti-adware/spyware program, surf the internet from a limited account, maybe disable some unnecessary services, etc., probably 90% of all the threats on the internet would go away. For somebody truly good all of the above would just be speed bumps, but the people who really know there stuff are usually not interested in somebody's home computer. Denial of services for web sites and mass mailing of spam from 'bot' or 'zombie' compters would probably be at least reduced if not almost eliminated. And you could still use an OS windows) that is actually compatible with scanners and printers. Just a thought.