The reason i quoted Chris' post was cause he didn't specify the wall of the RHS that would make it heavier. As I understand RHS and ERW have the same density.
So the question I didn't ask (I plead lack of sleep), Is it the RHS thats heavier or more dense or is it because of the wall thickness that the RHS would be heavier and more likely to break than deform?
However at the end of the day the above is all academic as I am still hunting down a supplier for ERW, there has to be a couple of them here in SA.
Edit to add: Does this appear to be the right stuff?
http://www.robor.co.za/cold_rolled_tube.htm