ok, first attempt using sketchup 8

[Jonathan]

Once you include torsional stiffness in the calculation the single box section is much better than RSJ:



The problem with either of these options is mounting the rails with sufficient accuracy. I doubt either steel RHS or RSJ will we sufficently accurate to mount the two rails on opposite faces, since the variation in the 'height' of the section will change the spacing of the rails leading to binding. Seems a bit risky... could fix it with shims I guess but you don't want to go there.
Instead I'd use two pieces of the 100x60 box section with one rail on each as that will be far stronger than just one and solves the problem of maintaining rail spacing.