Ideally both would matter. The higher the power, the slower the kick meter. The higher the accuracy, the straighter the kick, with less drift.
I'm not advocating taking away control from the user, I feel that the user shouldn't be able to outplay the ratings.
Difficulty of target passing does not even factor in for me. It could be the easiest or toughest mechanic in the game but QB accuracy should play into the outcome of it isn't sim.