I feel more impressed with Alabama's but it could be just because it is the more recent run.
In the three years for each:
Alabama beat more ranked opponents
Nebraska didn't lose at all.
Both teams had impressive performances in Bowl Games in their second and third years but shaky or less dominating wins in their first NC year.
I happen to think Bama's is the better of the two dynasties just because I respect the competition that they have faced more than the teams Nebraska faced. I think that It's much tougher to build a dynasty in a conference like the SEC where four other teams had won National Championships in the past decade or so (Auburn, LSU, Tennessee and Florida) than it is in the Big 8 where the only real consistent competition came from OU and only occasionally from Colorado.
We won our 1st with our backup qb. We may not have played as many ranked teams, but those games were not close either. But, like I said, I'm willing to conceded that they are currently equal, as long as it is acknoledged. ESPN did so this morning (it may have been a replay of last night's show), but this is a "what have you done for me lately" business. I mean, if Saban goes 8-4 the next two years, he'll be on the hot seat.