Short answer: #1 D.
One thing I think is overlooked is how much harder it is to field an historic #1 Defense in 2013/4 than it is to field an historic #1 Offense.
People talk about the '85 Bears and say we'll never equal them. What goes unsaid is that the 85 Bears could crush a receiver coming over the middle, pancake a back, and take down a QB without worrying about slapping his helmet, or otherwise contacting his head, neck, knees or ankles. In 2013/4, receivers look for--and often get--a holding/PI flag on every play. It's become a big soccer game out there.
We have overcome this, and being the most penalized team in the NFL, to field an historic defense. I mean, does anybody really respect how difficult it is for an offense to CONSTANTLY lean on its defense in a year; even more so when flag-happy officials are eager to penalize us and give opposing teams one fresh set of downs after another?
To the point of offense, it's no secret that teams are passing--and completing--more throws since the onset of these new pass-friendly rules. What Peyton Manning and the Broncos have done deserves real credit, but not without the disclaimer that it only happened AFTER it became virtually illegal to play a physical style of defense. Peyton was great before this, but only achieved legendary status once the league was softened up to start protecting QBs and the passing game. That hardly seems fair given the physicality and catch-at-all-costs style of play you see with guys like Wes Welker--he'll run headfirst into a wall or assassinate Aqib Talib, but blow him up and expect a drive-extending penalty.
It would be one thing if we happened to be the best D this year, but we are in the conversation for best defenses of all time--and may have won best secondary of all time. Does anyone appreciate how difficult it is to become historic on defense nowadays? And while it's still historic to break offensive records, that was bound to happen as the rules evolved. In short, it's harder to be an historic D than it is to be an historic O, and that's why I'll take the D.
One other thing in our favor: in the playoffs, officials have been calling very loose games. I recall John Clayton saying there were only 7 PI calls combined so far. I believe they will be even more sensitive to that, given what happened with the Seahawks-Steelers Superbowl. So in addition to losing home field (as do we) and the major altitude advantage, one more very important competitive advantage that the Broncos will lose THEIR "12th Man"--officials happy to throw a flag and extend drives.
Again, when the #1 O loses home field, altitude, and the officials--plus all the other qualifications--I'll take the #1 D any day.