[JA]

I did an earlier comparison of 100x100 Square Hollow with and without ribs against 150x150 Square Hollow on an equal added mass basis, handraulically...

100x100x6 Square Hollow Section:
Second Moment of Area = 3.04 x 10^6 mm^4

100x100x6 Square Hollow Section PLUS 50x6 Full length Ribs at 12, 3, 6 and 9 o'clock:
Second Moment of Area = 6.54 x 10^6 mm^4

There are four 50x6mm ribs used above. Instead of using them as ribs imagine adding each of these (distributing their mass) to each 100mm side of the 100mmx100mm SHS, whilst still maintaing the 6mm wall thickness. Well each side wall is now 150mm - You now have 150x150mm SHS. Now look at the Second Moment of Area of the 150x150mm SHS:

150x150x6 Square Hollow Section:
Second Moment of Area = 11.3 x 10^6 mm^4

So all other things being equal, the 150x150 SHS should have 6.54/11.3 = 58% of the deflection / slope / bending stress of the 100x100mm SHS reinforced with full length ribs. A 42% reduction.

In short, whatever gets the mass out furthest from the neutral axis in bending, will always win the deflection/rotation/bending stress battle (without changing the material of course)

Best
JA