I'm considering 90 x 150mm RSJ over 120 x 80mm 4mm Box; partly as I have 4 meters of RSJ and no box but mainly as my brain tells me 90x150 RSJ wouldn't flex like Box Section would ... but the spreadsheet says otherwise
Me to, I didn't see a RSJ option in the sheet, did you use the I beam one? :whistling:

RSJ's have deeper flanges :naughty: than the webbs and therefore the majority of the mass is at the extremities of the section. Changing the values will change the Ixx value and therefore the deflection. If you stand back and think about it the rsj is slightly deeper and nearly a third (box 10kg, rsj 16kg) heavier in weight so it will deflect less. But the weight is more likley to be a problem for CNC

For a purely vertical load then an RSJ is the most efficient beam type there is, It just for a gantry most of the load is on the side and its poor torsion qualities let it down.

Without knowing the size or intended use of your machine it is pure speculation as the best options and problems you face.:whistling:

Best start that thred and let us know........