BTW -- Just to be perfectly clear -- Personally there's no way I'd buy a gas truck... I tow way too much and I want the full monty deal of being able to blow past people while pulling the grapevine!
BUT -- If someone is asking about a suitable truck and states they only tow very occasionally... and lives in Texas (pretty flat)... then I can't see much of an argument for selling him a diesel. The added expense.. up front and per gallon just doesn't make much sense for the "power".
My Brother in Laws company switched their entire fleet to gas last year... and they're in the heavy equipment business (Pape)... while he would prefer to have the diesel... he'll be the first to tell you his gas truck is just "fine".