Was any testing done on solid sheets of aluminum?
My understanding is that the intent is to dump heat as fast as possible into the pizza crust. If that's the case, aluminum has a thermal conductivity 10X that of steel. The volumetric heat capacity of steel is about twice that of aluminum, but doubling the sheet thickness would still give you a sheet that's half the weight.
Cost wise, a carbon steel sheet is going to be the cheapest, but do the performance gains (i.e. better heat transfer, corrosion resistance, lighter) justify the additional expense?