Fact: *nix is more secure by design than windows. I do not think anyone disputes this.
Fact: All of the OS have become more hardened.
Wager: While it does appear that while the guts of Windows is less secure, Windows (Microsoft) has done a much better job of creating a good wall around its guts.
Wager: All the browsers have a lot of vulnerabilities. (Chrome has done pretty well though)
So where does that lead us? In real world practice I believe that windows is still less secure. This is due to install base, 3 party apps, (just looks at adobe reader 9 and less), years of built up skills and tools for hacking windows, and a Mac OS that is far better security wise than Windows XP. I am not willing to say that the current Mac OS is better or worse that windows 7. I like both of them. Neither are perfect. In some ways IE is more secure than Safari. But where I work I just can not get completely away from active x controls and the like. So if the hackers were starting from scratch, and they were up against windows 7 and OS X, I think it would be a draw. If I add in IE and Safari then I think that Safari loses. But if I use Chrome, well then - who knows!