Which metal would flex more?

[Zeeflyboy]

Would depend on the aluminium alloy to some extent but Youngs modulus of steel is roughly 3x that of Aluminium.

There is not really a particularly significant difference in modulus between stainless/mild.

Flexural modulus is usually fairly in line with Youngs modulus in metals. Polymers can vary more.

https://www.engineeringtoolbox.com/y...lus-d_773.html

Edit - as it pertains to machine design, the general rule of thumb I've seen bandied around is that you need twice the thickness of aluminium to achieve similar stiffness to steel. No idea where that rule of thumb started though nor how accurate it is when considering all the different variables in real applications.