Therefore, assuming the two scaff bars have the same wall thickness and diameter, the aluminium bar will deform first.

Aha - Steve's strut loads table is very useful! I'm assuming that "axial load" is what I was asking about? So very short tubes are stronger if made of ali - but for over 0.8 m lengths, steel is more resistant to bending than ali. Have I understood that right?I note that the first section on properties gives a higher "allowable bending moment" for aluminium than steel.