when i ran a landscaping crew, our 250 gas (single cab 2wd) got terrible gas mileage - about half of what our 450 got. so if youre driving it more than 100 miles per week, the deisel motor EASILY pays for itself.
we ended up selling the gas truck and getting a chevy 3500 with the duramax. and saw incredible profit increases on longer driving jobs. crazy how fast that junk would add up. especially pulling a trailer.
if you plan on pulling a boat or any trailer, the deisel might lose 10% of its fuel economy where the gas truck will pretty much cut in half.
in addition to all that, the deisel will have MUCH more of a resale value if/when you decide to get rid of it.
one more thing... if your buying new, look way out of the area. turns out that 3500 we got was priced so much lower in atlanta that we flew one guy out there, and had him drive it back and still saved over 1000 off of what we wouldve payed here.