Steel Frame
Pre galvanized, light steel that can be roll formed (cold formed) to suitable profile shapes can be easily fabricated (cutting and punching) and assembled with self tapping screw fasteners. The profile shapes can provide integrated continuous locking system (not quite as sophisticated as an aluminum extrusion) and so avoids the expense and labor of attaching separate locking strips.
The problem remains, however that the light steel should not be directly exposed to the Liquid Solar and Bubble Tech operations in the cavity space. This means that the Cavity Space is more complex to build as a closed duct like panel - however, this is still a reasonable solution if the Sola Fabric production is performed by a capable fabricator. After all that effort, the steel is still not good for more than 20 years, whereas the aluminum is good "for ever".
It depends on whether you have a roll forming plant that can be set up for volume production. In limited runs the extrusion of aluminum has lower set up cost and small production run minimums by comparison. 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.