Just my opinion, I would look at the milage towing and not towing. If I am driving 20k a year and only towing 4 to 5k a year, not sure I would go diesel. Yes the deisel will give you better milage towing, yes it pulls better. The new gas trucks, the dodge with the 6.4 for example can tow a lot and come in the 3500 series if payload is an issue.
I do not mind driving my diesel as a daily driver, some people do. If you are not fond of it and most of your miles are not hauling, gas truck will work just fine. You will be in the same payload issue with a gaser as with a diesel, the gaser will actually have more payload in most cases in the same series truck.
You can do some math as well, 5000 miles at 8mpg compared to 5000 miles at 12mpg, then factor in the price differance in the fuel. The price of owning a truck, when comparing apples to apples based on milage and trade in values, gas and diesel are a wash at the end of the day. I have seen no data to prove otherwise.