The outside dimensions give a gantry beam most of its stiffness, especially the vertical. And steel is a lot stiffer than aluminum. A box beam is stiffer than a T-slot extrusion, since it's the outer skin that counts the most in resisting flexure. So if your design has room for a beam with a 90 x 180 section, a box beam of that dimension would work best. The steel is going to be heavier than the aluminum, which is something to consider, since the momentum will work against you, especially when changing directions. Filling it with concrete grout would make it a lot heavier. Interior plates would be preferable.