If your going to work the truck, diesel makes sense all day long. If your just going to Walmart... gas will be cheaper overall. I think it's crazy to buy new anyhow, but I'm not always of the same mindset as everyone else. Diesel will cost more to buy, PM is more expensive, mileage is really what you buy and how you use it...varies too much for me to just blanket statement it. We have a fair number of 3/4 ton diesel grocery getters around here because they think its "cool", I'm ok with that...but not how I want to invest.
If you work the truck, you most likely will never want another gas model after that first diesel. Loaded truck mileage would blow away gas models and not over work it a bit... I like them, I just wouldn't use it hard enough to a good investment. Currently my 1000 dollar pos 98 3/4 ton chevy needs injector work... but it is the beater truck I need at a price I wont care about. Somebody else who is going to do miles wouldn't want it, but for the around the house crap I do with it its just fine.