Sorry—I’ve used both. Safari sux. Firefox puts Safari in the shade in so many ways. Safari isn’t better in any way.
Safari launches faster and uses less memory. Firefox has known problems with text rendering. And in my opinion, Safari has a better user interface, better typography, and it's web page rendering just looks better - especially during extended usage.
I do use Firefox on rare occasions where Safari has a problem. The Safari 3 beta seems to fix some of the old problems, so my usage of Firefox will probably continue to decline.