I've got 4 sheets of 16ga galvanized already at the house (cheap = free). I'll be needing 4 sheets of probably 18-20ga aluminum to finish the job. Which one should I do the inside with, which one the exterior.

I like the strength of the galvanized over the aluminum for an exterior covering. Since it's going to be a mobile oven (small garage and will need moved constantly for space requirements) having a harder material on the exterior would prevent it from looking like something my neighbor would drag into his front yard. On the other hand, having the harder material inside would be better from a longevity standpoint exposed to heat. Plus the aluminum on the outside would be shiny (pretty for the wife to see in the garage)

Anyone have other ideas? I'm not stuck on using either/or, and I'd have no problem buying 5 more sheets of galvanized, or buying 9 sheets of aluminum or lighter ga steel.

If it matters, internal 3x3x6'/4x4x7' external oven, it'll be riveted over 6" steel stud channel and 4" steel studs set alternating inside/outside to insulate the inside from the outside (prevent heat transfer to the outside walls), 4ea 3300W elements, 4way contactor, recycled oven electronic timer/thermostat...pics once I'm done....unless the house burns down around it.