Brass is preferred even though it is the pricier choice.

ARs have tighter tolerances than AKs and since steel isn't as ductile as brass you can get more fouling in the chamber due to escaping gases depositing gunk there, I was told just to put a brass case every 10th rd or so in your mag to keep buildup from occurring, most steel cases come from Russia and they run dirty.

The steel in steel cases are much softer than gun metal and won't themselves damage a gun. AKs love them Wolfs, never had any problems shooting them.

Handguns might be a different story with buildup, but if you clean like you're supposed to I don't see it being a problem either, With Glocks they say don't shoot brass after steel without cleaning in between, that's all I've heard mentioned of cautions.