Is one safer? Will the harder AR ricoche more or is it a concern for the metal deforming?
YES!
AR-500 armor plate will not crater even with high-power centerfire rounds at 100 yards.
When steel is correctly hung- meaning from the back side of the plate, so that it angle slightly "forward" at the top, bullets/frags are directed downward into the dirt, harmlessly.
Bullets will not ricochet if the steel is hung in this manner, and swings freely to absorb the kinetic energy of the bullet strike.
Mild steel WILL crater. And cratered steel, is dangerous- because when a bullet strikes the cratered area, it will ricochet off in some indeterminable direction- which could be back at you.
I've never understood why some guys insist on shooting mild steel. AR-500 plate is not expensive when you consider that correctly used, it'll last darn near forever. I've got some from 4" to 12" (I use them at about 2 moa in size based on distance) that I've had for five years, not a single "ding" or crater after thousands of rounds.
Jake has great steel:
http://www.jcsteeltargets.com/
I'll never forget the look on the postman's face when he "delivered" about 50 lbs. worth of targets in a large "FLAT RATE" box held together with about a million wraps of package tape! I bet USPS never guessed so much weight could be crammed into that box