I made the switch from a 2002 5.9 gasser to a 2004 duramax (2000 cummins work truck) i tow a lot, more than 50% of my miles are towing and most of my miles are highway, my gasser could tow the loads but being a 5.9 i would get like 4.5-8 mpg depending on what i was hauling, my diesels get 16-21 mpg depending on the load, the savings in fuel alone 20k+ miles/year make up for diesel being more expensive, the payment being more and the increased maintenance for the diesel. and last month i pocketed 50 bucks on fuel savings, straight to the mod fund.
The jist, if you tow a lot and want the maximum truck life and mpgs and power and can deal with added maintenance responsibilities and costs get a diesel