...unless you're going to do it like the old-timers or indoor duration types.

Once a model gets very big, foam does get heavier than balsa -- how light your building techniques are determines how small a plane that equal-weight cross-over happens at. The drawback is that leightweight wooden structures take a lot of work, both at drawing-board time and at building time.

Designing a lightweight round fuselage involves doing a very light truss frame inner structure, then adding formers and planking outside of that. (You have to lay up the outer skin by alternating sides as you add the planking so the stiffness of the skin doesn't warp the frame underneath.) If you make the curved outer surface the actual load bearing structure, the end product can be way lighter than you'd expect.