My two cents.....
Regarding steel vs aluminum, both tanks are certified for twelve years. If the steel tank is less or equal to half the price of an aluminum, it is an acceptable value to buy the steel. If the steel lasts longer than six years then you are ahead of the game. I suspect that if you wash the steel tank and give it a coat of paint every couple of years, it will last all of twelve years.
Yes, aluminum is nice, but I don't see the value in double the price or more.
BTW, the fiberglass tanks that you can see through?.... Because they are not made of METAL, they are only certified for FIVE years. Then you either have to get them re-certified or toss them and buy another one. If you really need to see into them then they are the only choice, but otherwise you could spend your money somewhat wiser, IMHO.