If you want to stick with gas go for the Chevy 6.0. Fantastic motor with decent towing power. If you decide to go diesel do yourself a favor and go with the Ram and Cummins motor. I’d also recommend finding a model with the Aisin transmission.
We’ve hauled at max capacity for a lot of years and put 150,000 miles on a brand new truck every year from 2014-2017. We owned two brand new chevy 3500s in 2014 and 2015 and two brand new Ram 3500s in 2016 and 2017.
We have officially made the switch to Ram from now on after being a Chevy and GMC hauling family since the late 90s. The Ram is an all around better hauling truck. Less needy on maintenance and you only have to change the oil and service it every 15,000 miles which is great when you are hauling as many miles as we do.
Oil changes and fuel filter changes every 15,000 miles and just drive the thing. It’s like it wants to be worked hard and never gives us any issues. Unfortunately cannot say the same for the 2015+ Chevys. They changed their turbo design and we had two turbos go out on us on our 2015. They are also notorious for injector and fuel pump issues which is very expensive repairs.