I was wondering this myself, I think box section looks to be a better choice than extrusion because it's cheaper and you always have the option of pouring concrete in to improve stiffness at a later date.

I'm still a little cautious about using ally in place of steel though because it is inherently a less rigid material, if you need to triple the amount of material to get the same stiffness as steel, the cost/weight benefit has just disappeared.