Safari is pretty fast and pre coded to operate efficiently within OS X, (it is an Apple application after all). But there are some issues with Safari from a security standpoint that eliminates it as an option for any online transaction where security or monetary protection is a concern. And though it has a lot of nice features, it lags behind other browsers in that respect.
Firefox is also fast, but not as pre coded as Safari is for Mac OS X use. It is full of features, (plug ins and themes), and very secured. The last update of Firefox 3, (3.0.1), however has been a little buggy on my Mac. Mozilla is presently working on version 3.1, (in alpha and soon to be released to beta for testing), that promises several levels of performance and enhancements.
i used firefox way back on pc, switched too safari when i got a mac, and back to firefox when v3 came out. i tried opera, but could never figure out how to import my ff bookmarks, so gave up on it, but my friend loves it.
they are all free.
download them all and see which works best for you.