Wood
While we could argue for days and days how well they fly, in my humble opinion and generally speaking the Great Planes balsa and plywood were of significantly better quality than Sig's. I always felt the Sig people just reached into the wood box for the "next piece" to cut or die cut. Often the two elevator pieces were of different weight balsa as were the ailerone stock ! Plus a lot of "twisted/warped" lite ply. I always wondered how they got it so bowed !
Bill