DEPENDS,
I usually go for the cheapest material but weight ALWAYS maters. Recently I have been making electric planes from de-papered $1.00 Handi Board from the dollar store.
The weight reduction is near 50% with the paper off. This has been a common practice with the "Lecky" scratch builders for some time. If its not actually 3mm or 4mm Depron its a very close second! Its the white foam paper covered poster board.

Just get it wet, spray with a common household cleaner,rewet if any dry areas visable, bag in a tash bag for a few hours or overnight and the paper peels right off.
It is very close to the same thickness as fan fold but is a nice solid white that is easy to color with felt tip pens. Cuts very nicely with a fresh exacto blade.

Balsa has a better compression performance than foam but foam can be MUCH less expensive.