Aluminum gas tanks (polished or brushed) are beautiful.......until you "lay it down" or otherwise dent or mar the surface, then they never look right again.

At least they never rust. It's a great aftermarket option for those who take this into consideration; the current prices make it more of a gamble than I'm willing to take.
Steel tanks on the other hand, are tougher, can be easily repaired by welding/brazing, dents can be filled with Bondo and repainted. But they also rust. I'm currently rebuilding a 1976 Yamaha RD400 and I've got this chore ahead of me.

Stainless Steel is a good idea; but it's more expensive than Aluminum. Just my two cents......James.