I think it depends on what you expect from your browser.
They are similar in many ways, but what made me to use FF were:
- I think FF is closer to W3C recommendation in interpreting the website's code,
- has lots of plugins, including Web Developer toolkit, Google Toolbar,
- has more skins to select from
Although I still have Opera and use it from time to time (e.g. to see how it displays some sites).
Hey, try it out, what have you got to loose?