Perhaps it's me who did not recall which season those two NFC teams played. My apologies.
I saw those Bill Walsh 49'ers teams--an outgrowth of Paul Brown's direct edict to Bill Walsh to redesign the offense to accommodate the Bengals' QB (Ken Anderson) who could no longer throw deep. I lived through his realization that there was a very exploitable window for obtaining players with more mobile and less powerful/stout athletic profiles at a great discount. I vividly remember how they utilized spacing so very differently than the rest of the league. I saw that plan hatch, grow, and blossom. Bill Walsh is rightly chided for calling himself a genius ... but he was right. The system he invented, tweaked, and masterminded was truly revolutionary. The concepts of spacing, running horizontally far more than vertically, and avoiding moving the DL's or blocking the LB's was truly ground-breaking. The only revolutionary part of the Bears was the entire overload blitz schemes that became a staple of every defense.
There was a lot more to figure out with the Walsh offensive system than there was for the Buddy Ryan defensive system--but both teams greatness is necessarily predicated on those coaching tactical innovations. To attack the Bears defense, you needed certain qualities, but mainly a different tactical approach, such as more pass protecting RB's, a QB capable of getting the ball out of his hands quickly, and WR's who could read defenses to stay in sync with their QB's. To defend the 49'ers offense you needed physical qualities that were not generally present in the players of that day--30 lb's less muscle and far more speed, great change of direction/lateral quickness, LB's who could see and ride out cut blocks rather than stack and shed power blockers, DL who were mobile rather than powerful, DB's who understood the horizontal routes and could tackle immediately before allowing any YAC. A front 7 who were versatile enough to hide their blitzes because everyone was capable of shooting a gap or executing a stunt/twist to become a pass rusher. Then, once you had those fundamentally different personnel--who would be exploited for lack of power by every other team in the league--you had to teach them an entirely different style of play.
It took the NFL many years to catch up. When it did, you saw teams with old-school coaches like the Harbaughs in Baltimore and San Fran build an offense based on the old power and fullback principles and no team in the NFL had the personnel to match up against them, because they had shifted their roster to defend the Walsh principles.
And while it was harder to defend the Walsh west coast offense system ... in time, it could be done. Just like a team could attack the Ryan overload blitz schemes. It's hard to say who is better from a snapshot, but that is one valid way to look at it. Another might be to try to remove the innovation of those systems and look at the players who executed it, and ask which was greater once the league had figured it out and adapted. That's where I'm coming from I guess ... trying to remove the novelty/gimmic of these innovations and evaluating the team (coaching and players) who executed those innovative systems.
Hope that lends some clarity to my perspective. I see your very valid approach of simply looking at the snapshot--much more clear cut--and respect that view as well. I guess when I'm considering what I see as the greatest of all time in an Undefeated Season, I tend to take a different perspective when I consider which team was closest to going undefeated. Losing in week 1 takes you out of consideration, and not winning the Super Bowl eliminates you from even the discussion IMHO. I can see a greatest player not having a team achievement (e.g. Marino not having won a Super Bowl) because a team is greater than one player. But I can't give a team a top billing if they fail the most significant team achievement (e.g. the Patriots who didn't even win the Super Bowl).
For me, the innovation of the Bears, the overload blitz, was adjusted to by the league. The far more complex innovation of the 49'ers west coast offense took a lot longer to compensate and react to by the league because it fundamentally required different personnel. But, when the NFL did ... even that great 49'ers dynasty was over. Hope that clarifies things regarding my perspective.
