Steel- provided you're at a safe distance for the ammunition being shot.
It's reactive, and as the poster above stated it never gets old hearing it ring.
You know instantly if you hit, both by movement, and sound. It's best to size the plate relative to distance and accuracy desired (as in, two minute equals a 4" plate at 200 yards). Carry a can of paint, I paint the main plate fluorescent orange and then a bright white circle in the center for a bullseye. With your scope or spotting scope, it's easy to see the location of the hits on the plate.