I think it all depends on how he is graded by the pro scouts. A first round projection and I think he has to declare early. If he is projected as a 2nd or 3rd rounder then I think it becomes a much harder decision. Collins is someone who has been thought to have first round ability, but he hasn't really shown that this season. From a financial standpoint, coming back for another year in the hopes of getting into the first round could be worth another $4-$6 million in signing bonus and his rookie contract.
I think with any player, a lot has to do with their personality, their family situation, their goals, etc., so what might be right for one guy can be for someone else. I agree that Collins has not had the amazing breakout year he had hoped for. He has fewer sacks than McMullen, Freedom, or Valentine. While stats can be deceiving, I think he does need to work on finishing plays. (The fumble against Rutgers is a good example.) He is disruptive, but doesn't seem to have that extra flash to pull down a QB or back like Suh did. But, NFL drafts on potential more than accomplishment. I don't know if his potential is improved, or even better showcased, by sticking around a year.
As a Husker fan, I hope he comes back. But I hope he does well regardless of his decision.