GMC 2500 question

Today, went and looked at a 2015 GMC 2500 Duramax with the All Terrain package. I can't really tell if the All Terrain option change the payload or towing capacity. I'm hoping someone here can clarify what the real differences compared to just the SLT.
We are planning to change to an Open Range LT308BHS, which is supposedly "1/2 ton towable", but after going through all the numbers, I feel 3/4 ton is better way to go.

Thanks