I believe that super alloy plating provides more protection than steel plating and weighs about half as much less, while hard plating provides more protection than both steel and super alloy plating and weighs about six times as much as super alloy and three times as much as steel.
You can craft both steel and hard plating with 4 mechanics, a welder, and lumps of steel.
It isn’t really isn’t worth it to put armor plating anywhere but the front of your vehicle, as every piece of armor adds significant weight, causing significantly slower acceleration, lower top speed, and worse gas mileage.