The Saints are incredible. They round out the season at Washington, at Atlanta, hosting Dallas, hosting Tampa Bay, and at Carolina. I am not sure how any of those teams are going to beat them. Anything can happen, and there are always upsets that no one sees coming, but other than the Dallas game, I do not see any of those teams coming close to beating the Saints.
On the other side of the aisle, the Colts have a pretty easy schedule left, too. But they have not been as dominant as the Saints, and if you had to pick one to go undefeated I think you would absolutely pick the Saints. The Colts' last five games are host Tennessee, host Denver, at Jacksonville, host the Jets, and at Buffalo. Jacksonville on the road jumps out - that's probably where they lose if they lose at all. But the Jets are sneaky and Denver is pretty good, so I am not banking on the Colts going 16-0.
This would be a great Superbowl no matter what, but if they are both 16-0 and go to the Superbowl at 18-0, it will be awesome.