Aluminum requires more welding skill. So, from the fabricator's perspective it's a more challenging material.Steel of the same strength is thinner, but also heavier and I would have needed to re-model the tanks for the thinner material. So, I'm glad he was unfazed.
If I had to re-model these for the material change, I probably would have chucked the current design and opted for a two part solution, seat replacing rack with tank.
I had no idea where the time for that was going to come from, so the whole thing probably would have come to a screeching halt.