Well, I think it will go to three sets, but whoever wins I don't think the score will be that tight. It's interesting that Federer's last two wins indoors, while convincing, were not straight sets, and Novak usually came back right after Roger had given a crushing display of superiority; he took the second set in the Vienna final 6-1, but Novak managed to take the third set, and then at the Masters Cup even after bagelling Novak in the first set Federer was forced to go into a third eventually.
Also note that this is the first time they have played on outdoor hardcourt, and of the six matches they've played Novak has won the two played in outdoor conditions, at Wimbledon and in Rome. I think Roger will still take this and although it will most likely be a tough battle, I reckon he will manage to win decisively in the end.