I think this contest is David A's to lose at this point. To not win, I think one of two things would have to happen: 1) He would have to have a miserable performance next week, forgetting lyrics or cracking a note or two; or 2) He would have to have a very "humdrum" performance and David C would have to have an incredible performance that truly shows his uniqueness as a performer.
Now, all that said, I certainly feel that David C is the more capable, the more polished, and the more versatile performer of the two at this point in time.
So, why does my prediction still go to David A? I think his softer, more ballad-y style has greater appeal to a broader spectrum of ages and people in the United States than does David C's edgier rock style.
But in the end it makes no difference what I think. Next week we will find out who will be the next American Idol.