The Aluminum Frame uses extrusions and provide the best opportunity to build in value (which includes integral locking profile detail) without an added cost. I would always prefer to use aluminum for a Sola Roof. Look at glazing systems - curtain walls etc. - you only see them produced in aluminum; not steel. So considering that we are building a building envelope that will regenerate more than its cost per year in energy alone, then there should be no reason to not go with a "for ever" investment since not even an embodied energy criticism can be used to argue against a system that is actively displacing so much energy consumption and is even producing energy. Energy will not be scarce in the future, if we get onto the path to sustainable living. Therefore, since aluminum is the most abundant metal (Silica 75%, bauxite 12%) its price, as a commodity, in the future will be most affordable. Even now my analysis shows that aluminum is a more cost effective choice than light steel when we take into account shipping weight and durability. So if we are thinking long range we must think Al.