Rune, you are correct, the rudders are very similar in area. The Impact rudder is a little taller but narrower. The rudder design you show is the better choice over a fully sheeted, composite type, because the CG of the rudder is closer to the hinge line. This is important because inertial concerns are minimized this way, so the rudder will be far less prone to flutter. And should you decide to statically balance the rudder, it will take less weight to balance it. BTW- a statically balanced rudder will eliminate flutter, not just minimize the possibility

MattK