Nudo are who make Polymetal, which would be my preference. They have a product that is a compromise between the two, aluminum skin on polypropylene corrugations.
www.nudo.com/p_alumacorr.php?crumb=%3Ca+href%3D%27l_sign_panels .php%3Fitem%3Dproduct%27%3ESign+Panels
The advantage (to me) of the aluminum skin is that it comes prefinished with baked enamel. I see they have a new product called ValuMetal, similar to Polymetal without the fire rating in white only.
www.nudo.com/p_valumetal.php?crumb=%3Ca+href%3D%27l_sign_panels .php%3Fitem%3Dproduct%27%3ESign+Panel
Whichever be sure to seal the ends of the corrugations shut before it hits the road.