Diesel - Improved milage, and I see biodiesel winning over ethanol. Biodiesel is available here, at the pump, ethanol isn't.
Actually, you won't get that much better mileage out of a diesel than you would a gasoline engine. I've got a friend with a newer F250 Diesel and it only got 19.5 or so miles to the gallon- according to the fuel computer.
Plus, a diesel takes a whole lot more oil when you change it.
I am absolutely in love with my 1999 Chevy Silverado 1500. It's got the 4.8L engine, and has plenty of power for everything except heavier towing- manual rates it at 6000 lbs towing capacity. It's also got the Z-71 4x4 Package. My father just got a '99 Suburban with the 5.7 engine, and he gets 18.5 MPG on the highway, and it rides really, really, nice.
It gets about 20 MPG around town.
I don't think Toyota or Nissan make either of their trucks with diesels, but good luck finding a Toyota used. No one wants to get rid of them.
I have really nothing against Ford, but every Ford I test drove while looking for my truck had poor ride quality. Worse than my POS beatup S-10 Blazer. I was looking at a 2001 F-150, and I could have had it for $9000 cash, but the transmission had been removed and swapped for a new unit. I looked up that year model, and seems to me that they had some transmission problems. Some of the mid 90's Ford diesels apparently had transmission problems, too.
Good luck finding something that isn't way overpriced, beat to hell, or too new. I had a very difficult time finding a truck around the $10,000 and 2000 year model mark. The dealers don't like to keep anything over 7-8 years old, and used car lots love to pass off a POS. It may look nice, but be a turd underneath.
Oh, and Carfax is your friend. They don't catch everything, but they have a nice deal- a month of unlimited reports for $5 more than running a single report. And they saved me from a couple of polished turds.