Looks like you got lots of replies but I guess I'll add some more info on the subject since these big trucks are real popular around here and I know several people with diesel trucks. Currently I Have a 97 dodge cummins 4X4 dually 5 speed 4.10 gears and I get about 16mpg city/hyw milage combined, when I haul a 3000# camper and a 4000# boat I drop 1-2 mpg depending on how hard I drive. Speed limits around here are 75, High elevation, lots of big hills becaus of the rocky mountains. 130,000 miles only problems so far is a cracked steal fuel return line, and I had an ignition heater relay go bad because I was not starting the truck properly when I first got it. Both problems I fixed myself
A very good friend has the same truck except he has 3.54 gearsand his milage is 18mpg city/hyw his milage is 125,000 and only problems have been gear case on the front of the motor spung a pretty good leak but was fixed under warrenty and his fifth gear slid of the main shaft due to lugging the motor down to low in fifth gear, had tranni shop install new nut with loctite to fix.
My stepdad has a 2001 F250 powersmoke 2WD 3.73 gears and an auto (which is for sale) that has about 75,000 miles on it now, that has been in for repairs under warrenty at least 10 times that I know of. I don't remember all of the problems but some of them are throttle position sensor twice, glow plug relay twice, bad injector harness under the valve cover once, oil leaks from engine/turbo 2 times, tranni rebuild once. And this one kind of baffled me but the truck had a recall on the injectors in cylinders 7 and 8, Ford said they were to big and causing those cylinders to run hot and fail. I have never heard of running different sized injectors on 2 of the 8 in any engine? Any way the milage this truck got was rather shocking for a diesel, it was used daily for his electical business hauling probably 1000 to 2000 pounds in the bed and a 6X14 box trailer moderatly loaded and it averaged about 10mpg, worst of 9mpg and best of 12mpg and one time going from Albuquerque to Pheonix unloaded at 70mph it got i think 15 or 16.
Now by my recomendation he has just purchased a 2005 2500 dodge cummins 6 speed and hauling the same load, same driving as the powersmoke he is getting 14mpg and on the trip from oklahoma city to albuquerque when he went to pick it up I think he got 17mpg running 75 on average, and this mileage is with the tires under inflated by 25#s. Can't report on problems yet as the truck only has 4000 miles on it but he told me this is the best truck he has ever owned and is very pleased with his first dodge purchase. I think you can see why the ford is for sale and why he is driving a dodge now.
As for the chevy duramax I can only go by rumors, but I have heard that they get good mileage empty(around 18 to 20 ) and not so good loaded ( around 8 to 10), I have heard that they do have some problems but I can't remember what they are at the moment. However I think they have the most comfort of the three. I used to have a chevy with the 6.5 liter turbo diesel 4X4 3.73 gears and it got about 12mpg in town, 16 on the highway and 8 to 10 hauling my camper and boat, not to many problems but I only owned it for 9 months because it had no power when pulling and would run hot(230-245 degrees). the dodge can hardly tell that stuff is back there, even on 6% grades with above metioned load I have never had to pull it out of over drive and the hottest it has ever got was on that trip in 105 degree air temp with the A/C on was 200.
THIS IS WHAT I GENERALLY TELL PEOPLE
If you want confort and good empty milage DURACRAP
If you want tough suspension and body POWERSMOKE
If you want a mix of above and a bullet proof engine CUMMINS
CAN YOU TELL I'M PARTIAL TO CUMMINS
I'D RATHER BE CUMMIN THAN STOKIN
Really they all have there ups and downs it is just a matter of deciding wich ups and downs you want. Hope this info helps out. Sorry its so long.
Josh