Firefox has a lot of nice features, add-ons and themes. Easy to use if you are switching from IE.

Opera has a very sleak interface, is very straight-forward, easy to use, but may take some getting used to.

Personally I like both, and I have both. Why not download both and test them both out and see which you prefer? They are only small downloads after all.

Or you could throw everyone off and get the Windows version of Safari.