Stainless will probably stay prettier (I don't care but you might). As dwanner wrote it will last longer on salted roads. Stainless is more brittle so if it flexes it is more prone to crack (and can be weaker in the curves in the pipe). Not all stainless is the same. Stainless is a misnomer. It is really stain less meaning it is less likely to rust than other steel. It can still pit or rust some. It is harder to cut, drill, and weld. If you weld aluminized steel you are asking for it to rust at the weld.