Dennis:
I think the real answer is that nothing is simple. It could be that the better airfoil in a sheeted wing pays for the extra weight, it could be that a bit more or less weight makes a difference, but not as much as the normal variability in a contest, it could really be that different pilots are going to get more or less out of a really lightweight plane, etc.
I suspect that once you get back an inch or two from the leading edge the fine details of the airfoil aren't going to make a whole lot of difference, so because of this the sheeting vs. covered won't show in the flight characteristics. Then it becomes more of an issue of ease of finishing vs. weight, and robustness vs. weight, and all those other secondary considerations that don't matter once the plane is in the air, but have a very large bearing on whether or not the plane gets into the air in the first place, and whether it makes a good impression while it's on the ground.