Just a personal opinion here:
Steel if very strong, but is not very rigid by weight. Lumber is very rigid in cross section, but has a lower overall sheer value. This means in layperson speak: Steel will take more overall weight before failure but will
experience more deformation. Not good in a floor joist. Lumber will not take as great a load before failure as steel, but will not flex as much, which is important in a floor support. Research and compare modulus of elasticity.
Lumber, pound for pound, will outperform steel tube (to get the same rigidity, the walls have to be very thick adding weight and cost.)