Steel
OP, I see that you are getting a plate as opposed to the wheel rims. Good idea.
I'm not sure if mcosman on this site still custom cuts and ships AR500 plates but he did good by me a few years ago.
I got these 3/8" by 8" rounds for $25 each + shipping.
I would not shoot any type of mild steel with rifle unless it is sacrificial one time use and quite a distance out there (150+++).
Handgun targets could be mild but needs maintainence to make sure no ridges are present which could possibly send pieces back.
These were all shot at about 15 yds and are a mix of 1/2" and 3/8" mild steel and some 3/8" AR500. As you can see, 9mm pistol cal carbines will severely dent 2" pipe. We stopped the use of magnums and pistol cal carbines when realizing that the damage was too much to safely shoot these.
The spring plates were angled forward to deflect as much pistol frags as possible. You can see the downward spatter outline on the 2" pipe.
This rifle fire at 1/2" mild steel was an experiment (from behind steel barrier) to see how much penetration there is for various types of 7.62 X 39 and the 3 rounds at right were .223 Wolf/ LC and Barnaul IIRC. Even the plastic cored German (top left & center ) stuck in the steel.
AR500 / 550 and armor plate is the safest to shoot at but I think the distances given by the suppliers is primarily a liability distance given.