If you have the money and are going to get a new to you truck anyway. Get a diesel no question it is worth every penny of the 6-9k extra your going to pay. That being said if your not going to get the new truck and want to stick with the old one tow away buddy. I obviously am bias to diesels but for good reason they are great motors like Vettezuki said a maintained diesel will last you many years/miles.
Okay the big three thats totally up to you Dodge 6 cyl Cummins great truck, Arguably the best diesel motor of them all king cab offers the most room in the rear seats. The Ford 8 cyl Power stroke is good for off road but also offers the worst ride quality (kinda like a roller skate) for a DD my last choice and the trans is among the worst IMHO. The Chevy 8 cyl Duramax my choice for the best ride, best trans (Allison) and I just happen to be a Chevy guy. Oh and it dose great off road yes I use my truck. Oh yeah I almost forgot about towing I tow my parents 32 foot travel trailer like it doesn't even exist.
Best of luck to you whatever your choice.
Chuck