OK, from above post -- I never was good at math, but you get the idea. Divide the number of miles you get per gallon into the cost per gallon and determine exactly how much you are paying for fuel per mile gas, or diesel, or propane even. (Some vehicles run on propane .. not very common, but they do ... Schwan's trucks comes to mind immediately!
Then weigh all the other issues everyone else identified and then decide which is better for you, gas or diesel.
But random say one vehicle gets 15 mpg diesel, and the other gets 20 miles per gallon gas, you still need to figure the cost per mile, based upon the cost of the fuel.
Comment on the above two posts. My father-in-law's brother is a farmer, farms something like 1000 acres property he owns and tenant farms for others. The trucks he owns which transports the corn-beans-wheat from the field to the elevator are Ford 500's (gas). Now there's some "load" on the back of those trucks. They don't go fast, but they sure do move the load! They're fun to drive too!