My gut feeling is that Button is the faster driver of the two, at least when he is truly "on it". Rubens experience might help him in a long championship, but Jenson is getting rather experienced too. Both drivers are smooth, both are good in the rain, and Ross will give them both excellent strategies. Overall either one of them could win given a fast enough car, which I think they will have, at least early in the season.
I have a €10 bet placed on Jenson for winning the championship at 101/1 odds, so naturally I'm cheering for him... But I also have smaller bets placed on both Brawn drivers to win the Aussie GP (at 20/1 and 60/1 odds), so I won't be too upset if Rubens has the best start to the season.
I think Massa will win, though, the race and the championship.