If I remember correctly the knock on Manning was he didn't have as strong an arm as Leaf, but he was more polished. The knock on Leaf was he was not as polished as Manning but he had a canon for an arm.
Again I don't think this year will end up being a Manning/Leaf hit or bust situation, but both situations seem similar to me
Rodgers has a stronger arm, and has played in a system where he actually took the ball from under the center, but he is not as mobile as Smith and he is not very tall for an NFL QB.
Smith played in a system that was geared for quick fast strikes with the QB taking all of his snaps from the shotgun, but Smith does not have the same arm strength as Rodgers.