you know, I don't know for sure if we can ever answer this question. Because it wasn't the boy that was the true key--it was Voldemort's decision that led to the events. If Voldemort had picked Neville, I don't think it would have panned out as it did. (Well, obviously.) I'm not sure Neville's mother would have died for her son--not because she didn't love him that much, but because it would have been different circumstances. Supposing she hadn't just been with Neville? Supposing she had already put him to bed. Would she even have had time to get to the crib to shield Neville?

Again, we will never know for sure. But I do think that Neville would never have been able to shoulder that much responsibility. He is a wonderful character, don't get me wrong, but he would not have been able to take the weight of the world on his shoulders like Harry did.