Firefox because it's open-source and the community around it. While that lasts (and I see no end in sight), Firefox will always have the lead.
Also, what do you mean by 'faster'? The program itself - I'd say Firefox just by a bit, at least when I used Opera for a bit, which was when FF 1.5 was the latest. The web? There's no real difference, but with
FasterFox, Firefox can pre-fetch pages in advance...and while it may be frowned upon, I really like this feature.
Then again, I started with Firefox like many others, so I'm a bit biased.