My last 4 trucks have been diesels and if there was a hardcore advocate for the oil-burner I was it. With that said, I recently decided to trade in my 2006 Duramax LBZ on a 2008 Ford SuperDuty. After weeks of research on the forums and running the numbers (Cost of Diesel vs Gas, with mileage differences considered) I purchased a 2008 Crew Cab with the 362 HP 3 valve V10.
The truth is (in the U.S. at least) the EPA has ruined many of the advantages the diesel pickup once had. Ultra Low Sulfur diesel by itself lost approx. 10-15%+ of the fuel's efficiency no matter what truck you drive. The fuel sucks plain and simple. In addition, just refining the new ULS fuel bumped the price approx. $0.15-0.20 per gallon. This had nothing to do with the price of oil. Furthermore, as us sledders all know, the mileage of a diesel... and the power drops off even more with winter blend fuel, which is when many of us tow the most.
Starting halfway through 2007, the next tier in emissions requirements was implemented. Once again the fuel mileage decreased further and what was once a relatively simple motor is now a mechanics nightmare. You can hardly bomb out the motors anymore without losing your warranty and who wants to take that risk on one of these new emissions cluttered "Super Clean Motors".
To each their own but I get 11-12 MPG empty with my V10 (opposed to a reported 13-14 mpg "all around" with the new 6.4 Powerstroke) and towing my 27' Haulmark Enclosed trailer loaded to the max I get approx 8 mpg. That figure gets better if I tow on flat land but since I live in Alaska our terrain is far from flat. Factor in the $0.70 per gallon premium for diesel and the mileage advantage is all but gone.
As for towing performance, I can pull the same grades with the same loads as my previous Duramax and am typically within 5 mph of the exceptional Izuzu motor in the GMs. We're talking the difference between 70 mph and 75 mph pulling Thompson Pass out of Valdez. The big gasser does like to shift and rev more but I have never experienced a lack of power.
Lastly, I just got back from a 12 hour round trip hunting near Chicken, AK. and I drove up with an Uncle with a 6.0 Powerstroke. We were towing very comparable loads, in fact his enclosed trailer is also a 27' Haulmark. At our fill-ups we were within 1 to 2 dollars when we compared receipts. I burned more gallons of fuel but the higher cost of diesel offset his fuel mileage savings.
I know this is a long post but since I have extensive experience with both setups I thought I would share some real world data.
P.S. If you do consider getting a gasser, and you plan on towing heavy do not buy a 5.4 in the Superduty chassis.