According to this site[0], it's at 7.54%. Out of that, only 32% are running the latest version of OS X (10.9). Another 24.5% are running 10.8. So almost half of OS X users are at least 2 versions behind. Not nearly as up-to-date as you might think.
Well, it's a good thing then that you can't make a direct comparison between the two OSes. Apple doesn't care about developers, quickly deprecating APIs. Therefore, targeting an Apple OS that's 1.5 years old is much more tedious than targeting a Microsoft OS that's 1.5 years old.
That's in the US. In the rest of the world, it's around 5%.