First off, material softness / hardness has nothing to do with stiffness / flexibility, which has nothing to do with strength / weakness. Steel is 3 times as stiff as aluminum (not 2 X as you stated), because its modulus of elasticity (Young's modulus) is 3 times as much. This is the same for all grades (strengths) of steel or aluminum. Steel is also 3 times as heavy per volume as aluminum. The bending stiffness of a thin walled tube is linearly proportional to the thickness (t^1) and proportional to the cube of the diameter (D^3). The weight is linearly proportional to both the thickness and diameter. Therefore, it is theoretically possible to make a tube out of either material, which has the same weight and stiffness. If the diameters are the same and the steel tube is one third as thick, then they will have the same stiffnesss and weight. Oh, and for those of you who wish it bring up the D/t limit for local shell buckling, the D/t limit for steel is also 3 times that of aluminum, so it is theoretically possible to make a steel tube that is one third as thick as aluminum, without reaching the critical load for buckling. The proneness to denting of a steel tube with a thickness that is one third that of an aluminum tube with the same diameter is the same. From a practical standpoint, however, there is a limit to reliable drawing of thin walled tubing, which is why you don't see oversized steel tubing with super thin walls.