the aluminum top is better because it wont crack, but if anything it is not to prevent corrosion. it is anodized, but if that is ever worn away or eaten through or whatever, the aluminum will cause corrosion. all copper is a nice route to go, but the aluminum top is a good solution. i had one and dont mean to scare you away from it, but just wanted to clear that up. if you were choosing between aluminum and poly, go with the aluminum. if you are choosing between aluminum and copper, copper would be better, but its really expensive, and i dont know if it would really be worth it, but its up to you