Thanks m_c!
I have not heard about Fadal and David. I like the vertical toolchanger he has built. And how he explains his ATC drawbar.
But steel remains steel and cannot be compared with grey iron in machine tools :) . When he pushes the machine a bit harder it starts ringing...

Yes the online calculators I've used for the RHS deflection were crap. I have checked with a FEA and it gave 0.0081 mm for the 160x80x5 1 meter RHS with 50 kgf in the middle. Now compared with the 160x80 aluminium extrusion with 0.012 mm deflection, if it had the same weight (18 kg), it would deflect 0.009 mm.
In conclusion the rectangular hollow section steel is a bit stronger than the aluminium extrusion with the same external dimensions and weight.
And I think that steel has a little bit better vibration damping than aluminium but I'm unable to find figures.