Both browsers are quite good but Safari edges out for my usage. RSS integration and overall OS integration such as lookup in dictionary (lots of bloggers with their big words), mail, automator are all Godsends and Acidsearch plugin is also amazing.
Overall speed is also better in Safari, its really fast. However i love Firefox's save/restore session feature, spell check, clear private data on exit, force links to open new tab not window features AND you can customize the look.
Safari is quite possibly the ugliest browser i've had to use. UNO used to help but i recently deleted the app and now i can no longer re-install as their site has been suspended.
If firefox had an RSS reader integrated i'd switch to it faster than you can say cat in a hat BUT it doesn't. I also wish Safari wasn't such a memory hog and was as customizable as Firefox.
Oh well, i'll have to use both for now. You know what they say, two heads are better than one.