I would choose the slightly longer option. Ten thousandths additional length ought to be plenty.
Aluminum or carbon steel? - That's a tough call. Probably aluminum as it is one less thing to worry about rusting on you.
My son tells me that there are corrosion issues where dissimilar metals meet, which would favor carbon steel. I've never seen any corrosion on aluminum pillared rifles that I've owned though. - Rifles with the Hogue rubberized stocks for example.
Kind regards, charlesb