If you don't want to roll the fenders, then 20's are out of the question I believe. Also, chrome might be heavier, but it won't strain your tranny. Now, if you want to street-race (which I do not support) or do track time, and you want pure performance, then hit up with the aluminums. They're marginally lighter. Also, before you buy, make sure offset and everything is correct. The brake calipers and schtuff should all be considered.
My two cents. I'm not a professional mechanic, so some others may disagree.
My two cents. I'm not a professional mechanic, so some others may disagree.