It isn't that hard or soft material is better. It's a way of fine tuning your bow. For a RH shooter, a soft material for the strike plate, such as a rug rest, like a Martin rug rest, will move the point of impact to the right. A soft material for the shelf will move the point of impact down.
A medium material, such as Velcro, will start moving the point of impact back to the left/up. A hard material, such as plain leather, will move the POI further left/up.
My recommendation would be to stick with whatever material the bow comes with until you're sure your form is correct. It doesn't do any good to get a soft strike plate to move the POI to the right if the real reason you're hitting left is because you're not getting your elbow back behind the arrow at full draw.