I would think it would be Callahan. The dude had all the talent necessary to succeed and also came in when Nebraska was still fairly relevant, there was no excuse for the level of failure he had.
Pelini started off pretty strong, demonstrating the talent that existed on defense (such as Suh). He just could never put an offense together and by the time he had an offense that could move the ball a little, his defense started slacking quite a bit... well at least against anyone semi-decent. Plus once Callahans talent left, there was a noticeable drop-off in production and Pelini could never bring in that level of talent. The irony was that he was still a better coach in that he could still win more than Callahan even with less talent.
Thanks for the info. I only said that stuff because over on that other site some Huskers fans said they can't stand Pelini.