I would go by the weight of the trailer when fully loaded. If it is less than 10,000 lbs. the gas with the right gears will be OK. If it is more than 10,000 lbs. I would go with the diesel engine to get the greater torque for going up grades and accelerating into traffic and the downhill braking from the diesel engine's exhaust brake.
A gas truck may be tow rated for 12,000 lbs. but I would no dream of actually towing a load that is anywhere close to 100% of the rated maximum by the manufacturer. In the past I have found their claims to be far from accurate in terms of what was a safe load to tow.