Neither one will hurt anything, the patterns just suck and there will be a hole in the middle because the shot will tend to spin as it leaves the barrel sometimes. I guess shooting a lot of steel shot out of a rifled barrel may wear it out eventually, but the hardness of the steel shot is no where near as hard as the barrel in your gun. And considering the price of steel shot and the amount you would have to shoot through it to ruin it you would have to be pretty stupid to keep doing it. Especially considering the poor performance you would get out of it. If you were dumb enough to run that much steel shot through a rifled barrel you deserve to ruin it in my opinion.

Buckshot won't do anything other than maybe lead the grooves up after some time. And it would shoot like crap as well because there is no choke in a rifled barrel.

Paul