Well I think the B10 has a slight advantage in D (top 25) plus I think Teams like Pitt were ranked. I am not sure if B12 teams played other D ranked teams in non conf, but I know B10 did.
As for O rankings, I would hope when comparing a conf that passes more to one that is more balanced if not runs more, the passing conf would have more yards (remember yards dont equal points always), boosting their stats and rankings.
In the end that whole arguement will always come down to opinion, because you can find numbers that support either side.
As for which is easier to cover, you have to think some B10 teams are more run heavy, some more spread, some tried using running qb's. You name it and some teams in the B10 tried it. You can not say that about the B12. Since all B12 teams recruited and set up to stop the air attack, it would be rather easy to have even a modest running game and have sucess doing it, that would make that team look even better at running the ball. Nebraska vs Missery. Iowa vs Missery are good examples of this.
Recruiting and then covering the spread is much easier. Not to many passes go for under 3 to 4 yards, yet if you can gain that running, the chains keep moving. When you have RB's that are RB'S by vote and that team plays in your conf, how much do you need to worry about the run? So Missery was ranked in O and red zone D, but really, you knew they had no running game, so how hard are they to defend? Now not all B12 teams were as unbalanced as Missery, but they were far more unbalanced than almost all the B10 teams. So yes I would say it would be much easier to plan for and defend against the teams in the B12 compared to the teams in the B10.
This is no way a knock on NE, I think if you all had a gun slinger, you would have been one of the best balanced teams in the B12.