This is a good debate. Personally, I'd go with gunnery (or aim) as being far more important than "acm", especially for your typical MA flyer.
I personally think it is more important for everyone, but your "average" player in the MA doesn't engage without the odds being on his side, so good gunnery will ensure more kills and fewer assists.
Most of the "aces" in AH that have "amazing aim" really don't. For the sake of my ego, I'll just lump myself into that category and tell you why my hit percentage is high. I usually take shots I almost literally cannot miss. About the farthest I'll shoot is 400 yards, and that is rare. Typically, I'm at 200 yards or closer. At that kind of range it really doesn't matter to much what the other guy is doing (whether it is a deflection shot or not), I don't have to do to much in the way of leading.
Another thing that helps in developing your aim is flying with the same gun package for a long time. Leviathn has been flying the Spit 5 since dinosaurs walked the earth, so naturally he has a very good idea of how to hit different shots with the Hispano.
I have horrible aim with the Hispano, because I'm more used to the MG151. With the Hispano, I lead even though I really don't have to.