Which plywood via which plastic... The Tokai case my beloved Jazz came in, which is very much a Fender copy is something like 2.5mm plywood. A full on flight case might be 12mm. There's a hierarchy in cases really which ranges from "protect from scratches, I only ever handle it and gets its own car seat" through "travels in my car with the amp but carefully loaded by me" right through to "safe to be handled by minimum wage baggage handlers out of sight". Once you start getting into rigid materials I reckon there's so much variation it's hard to say one material is intrinsically better than another.