If you go with the diesel (I would), get an auxiliary engine brake because you will need it on I-40 and other NC-TN roads that are even worse. I have a D-250 and a D-350 both with the 12-valve 5.9 liter Cummins, and do not have Jake brakes. I stay out of the mountains.
I have a Dodge RAM 3/4 ton with the Hemi. I live in the California mountains. With 2 big horses (both 16.1+) I only trust my man to drive (he is a professional Hazmat driver with years of hauling experience with horses). With one horse no big deal. But unless you are SUPER good, and have just a light 2 horse trailer, I would step it up to the diesel. If you have a 3horse slant, or your horses are huge.... step up anyways.