Yeah, the solid would be stronger but not by much. The outside edge of the beam, furthest from the neutral point is where tension and compression are the highest (and where spreading over area comes in). Tension and compression diminish the closer to the neutral axis. I would guess by square law but I don't know.
Anyway the gory details don't make much difference in strengthening a forearm beyond a solid beam will add a lot of weight with little gain in stiffness.
