Fuel mileage, by itself, can never justify a vehicle switch. That said, if your vehicle is dead and you need another one, look at the fuel mileage as a factor. If all you're doing is replacing a serviceable vehicle, you need way more justification than fuel efficiency.
That goes for anything, not just a gas vs. diesel tow vehicle. Years ago, my sister was driving one of those 2-generations-ago Town Cars. She liked it, it drove OK, was comfortable and reliable, but got about 15 mpg max. With just her and one kid, it was too much car, but it was cheap to buy at the time she bought it. She started asking me about what else she could get that might be in the high 20s or low 30s for mpg around town. I asked her what was wrong with the Lincoln? She said "Nothing, I just can't keep putting gas in it."
I built a spreadsheet, where you put in the cost of a gallon of fuel, the mileage of car A, the mileage of car B, the income from selling car A, and the payout of buying car B, and it would show you the break-even time of the trade.
She kept the Lincoln, as no car she could afford was going to run the 12 years it would take to make it worthwhile to change based on fuel efficiency alone. You can buy a lot of gas for the thousands a vehicle costs.
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