I use FF 3 about 95% of the time, and Safari 4 beta the rest of the time. As mentioned, FF3 was leaps ahead of FF2 (especially on Mac) and the page zoom option (command + or command -) was a huge improvement over resizing text. Safari 4 added this feature, but the increments/jumps are twice that of FF, so sometimes page zoom on Safari 4 just doesn't result in the 'right' level, when you want the setting right in the middle. But, it is a step in the right direction for Safari. FF extensions are the big thing, and there are a lot of useful ones which can literally customize the entire browsing experience just to your own habits, needs. The first release of FF 3 did tend to crash quite a bit on OSX 10.5.5, usually when two-finger scrolling, but with recent updates in bot OSX and FF, it seems to have cured that. Safari renders pages a bit faster...
Get them both, use them both... one will fit you better as your browsing style evolves. All references to Safari are about version 4 (beta.) FF3 and Safari 4 are both a lot better than the current/(old) version of Safari, IMO.
Snowleopard2008: re: FF scrolling not being smooth... have you checked the option for "use smooth scrolling" in the preferences? It seems smooth to me with that option enabled. Also, about the instability...do you have all the latest updates installed? It might make a difference.