I like a hard surface to iron and press on. It has more stability and makes it easier to press. This may be an old wives tale but years ago I heard a tip about those metallic covers. The source stated that if you are not careful they can scorch the fabric because they reflect the heat back into the fabric and super heat it.
I have always liked using plain duck cloth for my covers with two layers of W&N batting under it and pulled very tight. It works for me.
I don't know if metallic covers actually will scorch fabric but the theory seems logical, and if you like to iron with a very hot setting how much are you really increasing the temp and the amount of time you leave the iron in place?
Oh! It just gets more confusing!
peace