The carpet comes standard with a poly backing ( clear plastic ) and the mass backing is a black rubberized material that is more durable and lets the carpet lay out better IMO .
I bought mine from rock auto and got the mass backing. Rock auto had the best price and the cheapest shipping. The mass backing helped the carpet keep its shape. I didn’t get it on the first car I did and regretted it.
You might also consider using a dynamat clone. I used the noico stuff from Amazon. It really quiets the car down.