That is a good question. I have been told by those that used to dust with J3's that a wood spar 3 would always carry a larger load than a metal spar 3. The biggest difference aerodynamically is that the wood spar LE stops well in front of the spar, allowing the fabric to dip way down between the ribs when sitting on the ground. But maybe it also ballons up higher in high lift situations?, and this "flexable" airfoil may actually be better? The wood spar Cubs are usually a little lighter, but not enough to make a real big difference in hopper load.
Also, maybe Wayne knows, Why did Piper keep useing the short LE outboard section on all the later J3's, PA11, and PA18. They went to the long LE on the J5, PA12/14, but it stayed short on the OB on the other models, there must be a reason. I asked this somewhere before, and no one seems to know.