RE: what to use for main spars?
I think a wing with some 'elasticity' in it is a good thing. Kind of like the suspension in a car. For models, I'm thinking more along the lines of crash resistance than flyability.
I know that for full size planes, it's critical to have a certain amount of springiness and flex in the wings, but this has little to do with how well the wings will survive a cartwheel landing.
The wingtips on the new airbus A380 flex upwards by several meters on takeoff and landing.