There is no easy answer to this one. Actually the stiffness of a brace will depend on a number of factors such as :-
material (and the term ally usually refers to aluminium alloy). Alloys of steel may include small amounts of chromium, molybdenum, carbon etc, etc. and the actual quantities will alter the characteristics of the alloy including corrosion resistance, life expectancy plus all the other properties! Aluminium alloys can include silicon, magnesium, copper etc.and again properties can be vastly different especially when heat treatments are taken into account.
Inside and outside diameters will also affect the performance of a brace as will the direction of the forces imposed on the brace. Just in case someone is interested as to how I know some of this I,ve taught metallurgy and A-level Physics.