Ok, steel vs ally for frames, the primary issue here will be spanning ability, this will be limited by the malleability of the profile you are using, angle iron in mild steel will be drawn and will have a very linear crystal structure, this leaves it quite weak in some directions, a similar thickness of ally angle will be extruded from a liquid, it cools in its extruded form and has effectively right angled crystalline structure which gives it an incredible structural rigidity for its weight and in comparison with folding sheet ally into a similar profile. So in conclusion, a similar thickness of steel 90 deg angle profile and its ally equivalent will have very similar spanning capabilities but the steel will be many times heavier also the structural stiffness will have very different properties, the steel will sag over a long run, the ally will either hold its load or fail catastrophically. My personal preference is for ally for lighting fixtures because you can always triangulate and use multiple lengths to add rigidity without excessive weight cost. In my experience the cost of a 16 gauge angle profile is very similar for both materials, ally extrusion can be bought cheaply from home despot type shops as 'picture frame' very cheaply in 8 foot lengths.
