I opted not to get the diesel. Yes, they do get about 15% better mileage but, diesel up here costs 20% more than regular gasoline.
Besides, I can always rob fuel for the dirtbikes or boat and vice versa if the truck is desperate for gas in the middle of the desert somewhere!!
Last time I drove down to cali I drove 70-75mph not towing the boat and got an average of 16.5 mpg.
My boat is very light and the 6.0 gas motor does a steller job.
On the other hand if your hauling a 30' Eliminator around, the Duramax is probably the way to go.
Food for thought,
Wicky