I would expect the stick ribs to be lighter than equally sized birch ply ribs. After all, spruce is lighter than birch. The big difference though is are the ply ribs going to be the same size as the stick ribs? I would expect they need a bit more material to make up for the weakness of the grain direction not matching the direction of each piece. With stick built wings, most pieces will have the stess running pretty much from one end to the other, parallel to the grain. With plywood ribs I would expect they may need a bit more heft in some areas to get that same strength.