When building a plane using anything from balsa to plywood, which is best to use in what manner?

I know the T-34 uses balsa framing with balsa sheeting and ply cross-sections of the fuselage, but the Mach 2 uses ply frame and cross-sections with no sheeting and its lighter.

The Lanier (<< I think) Viper has foam core wings with balsa sheeting making it extremely sturdy, but are built-up wings lighter?

The Mach2 has a very square shape perhaps due to the use of plywood, on the other hand the Pheonix Su-31 uses a lot of balsa and has a smooth shape.

Basically what materials used in what fashion makes a plane sturdy, smooth, or light.

I'm not into using Carbon fiber and fiberglass for base construction.