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The material doesn't matter at all - moly isn't less bendy than mild steel and alum is worthless.
Wall thickness is what's left, and it doesn't do much (although it's cheap and there's no down-side). You don't even really need it hardened or chromed if you have bushings (needle bearings or plain rocker ID: absolutely necessary).




Modulus of elasticity for steel is around 29, (10x6 psi)
You are right, alum will bend easy 10, (10x6 psi) titanium is a little better with 15.5
A common misunderstanding, is that a molly chassi is stiffer than a mild steel, but the modulus is about the same and with thinner walls, the molly tubes will flex( bend more then the same diameter mild steel). Stretch as in permanent deformation, is what cr-mo is good for.