John Gruber with a great comparison of Firefox 3 and Safari 3 beta browsers for the Mac. His preference is for Safari 3, though he notes that Firefox has some important advantages.
I've been using these two browsers (Firefox 3 b5) for a few weeks now as well. I prefer Firefox 3, for a few reasons. As Gruber notes, Safari is a memory hog, and given the number of applications I have open at once, Firefox's efficient memory usage makes a difference. I hate that Safari doesn't offer that option to open up with the tabs from your last session. Such a simple fix, I have no idea why they haven't added that after so many generations now.
But Gruber is also right in that both are a step up from the previous generation: faster, more powerful and functional.
UPDATE: According to ZDNet, Firefox 3.0 b5 holds a slight speed edge on Safari 3.1, though both are faster than their previous versions, Firefox 18.104.22.168 and Safari 3.0.4.