There are roughly 60,000 known viruses for Windows, roughly 40 for the Mac, perhaps 40 or so for Linux, and (very roughly) 5 or so known for commercial UNIX platforms.
Doesn't that contradict the title (not just a numbers game)? Well, no, and if you care (most don't) contact me and I'll be happy to go into the gory details about why Mac OSX and *NIX platforms (and Linux) are inherently more secure. Will this change with Windows 7? For the sake of the billions lost each year in productivity due to viruses and worms by companies that use Windows, I hope so.
But, just looking at the numbers, if you want 'safety' you can draw your own conclusions about what type of computer is going to be safer. Doesn't matter whether or not the numbers reflect market share.