I won't try to argue the Pats should be favorites, but here's a few things that may help:
1) Despite how much better the Bills looked in the last game, they were a JC Jackson dropped INT (which is rare when he gets his hands on it) away from having the ball back late in the 4th down 1 score, with the offense having scored on it's last two possessions I think.
2) The Miami game needs to be taken with a grain of salt. Much better Patriots teams have lost to much worse Dolphin teams historically down there. It sucks that we lost, and there were some things that pissed me off about how it happened, but I almost chuck those games out when trying to evaluate this team every year, because it's the freakin' twilight zone down there.
3) I don't think the Bills are chokers by any means. But the playoffs are almost a different game, and you just don't know how some players will/won't react to the atmosphere and pressure. This isn't to say that should make you think the Pats are going to automatically play better, but that once you get in the dance, unless you're someone like Tom Brady who has a massive track record of coming through, it really is "Everyone is 0-0".