This one's easy...while you can disguise a Bad D-line with linebacker scheming and a good secondary, you can't really do anything to disguise a Bad O-line...it will ruin both your running game and your passing game. Also, if your O-line can't keep your offense in the game, your defense will get worn out and start giving up big plays.
I was going to say the opposite. You can disguise a bad O-line with lots of screens, bootlegs, and 3-step drops... but you can't disguise a bad defensive line. I'm probably biased by Denver last season- they had arguably the best O-Line and arguably the worst D-line in the league, and they gave up more points than they scored. At some point, you *HAVE* to be able to get pressure on the passer or you will lose every game.I guess if I had to choose for one unit to be average and the other to be terrible, I'd rather have an average offensive line and a terrible defensive line. On the other hand, if I had to choose for one unit to be exceptional and the other to be terrible,
I'd rather have a terrible offensive line and an exceptional defensive line.