pro: cheaper, more common, more flexible, easier to work with minimal hand tools, copper is a necessary yeast nutrient, good heat transfer
con: copper doesnt like extreme pH so no acid washing, it scratches and tarnishes easily, can form poisonous copper oxides if you let it (green patina)
Pro: very resistant to most chemicals, highly scratch resistant, high stregnth, looks 'professional', thin (which negates the '+heat transfer' point of copper)
con: tough to work with especially without power tools, tough to fix mistakes on, does not provide yeast with copper
i have both a copper and s.steel HERMS coils, and i prefer the steel. there is no easy way to physically clean inside the coil, so acid washing is the only way to remove the buildup. my copper HERMS looks like hell inside after only a year of use, and that is with flushing with boiling water + oxyclean after each brew. theres just no way around it.