The GM gas engines and its Duramax engines put our roughly the same horsepower but the diesel has nearly double the foot lb of torque. Big difference in acceleration, ability to pull a grade, and especially in engine braking (even without the exhaust brake enabled) on a downgrade. The diesels are also stronger (have to be) and so the maximum tow rating is higher for the diesel powered trucks.
With a trailer load over 8,000 lbs. I would want the diesel for these reasons, regardless of the tow capacity of a gas powered truck. The extra cost of the diesel engine and special transmission is part of the price to be paid for towing a heavy trailer.
