This wing is for a discus or tip launched glider. There is a definate line wnere weight has a negaitive or positive effect. Too light and it's hard to get max altitude, a mldestly higher weight transfers into momentum and speed at launch, and hopefully higher launch. If you have never seen or launched a glider this way, it's amazing the altitude you can get for the least amount of energy spent. My answer to the question would be polyspan on the bottom, and tissue on the top. Best of both worlds with punture resistance on the bottom when it DTs and lighter weight on the top of the wing. For for extra torsional stiffness if needed, you can dope on "X"s or diagonals of carbon fibre tow on the bottom and/or top of the wing. Or add internal diagonal bracing.
I WILL get around to building those kits you gave me, as this is an interesting way to get a glider in the air, even if you have an older, worn out arm!
Type at you later,
Dan McEntee