They say the steel cases are more rigid and are hard on the extractor... if shot a lot. I have no proof of it, but who wants to make his own AR a case in point? Massad Ayoob who is usually right on the money on most counts says don't.
I shoot steel every now and then when brass runs out.
You may find soon that this topic is inflammatory. People who shoot steel have called me names for what I just told you.