Not much of a contest.
Like asking, "Which do you prefer - $50 or a poke in the eye w/ a sharp stick?"
I used Opera for several yrs, until v9 came out. A lot of loyal Opera users returned to v8.5x because v9 had so many bugs. Even before that, it seemed like there was always something wrong, and it was hard to get answers - for me, at least. But then, I was using dial up, and Opera was so much faster than IE, it wasn't funny.
I've used Firefox since v1.5, and never really looked back. If you want (far) more resources for KB articles, etc., and better support forums, I'd say go w/ FF. If they've got Opera 9.x ironed out, it may be a little more of a "plug & play" than FF, but not by a lot.
Far more softwares say they support FF than Opera, but it's possible they'd still work w/ Opera - dunno.