Material alone does not determine stiffness. You can make a noodle or a brick out of any material, depending on the shape, diameter, thickness, and length of the tubes. Steel has different material properties than aluminum..in fact, as a material, it is much much stiffer than aluminum, stronger and more fatigue resistant. This is why steel frames have a greater range of ride characteristics than aluminum. Aluminum frames have to be stiff or they won't last very long.