Good choice on the duramax! Youll love it!
For work we have an 06' duramax 2500 with the 6 speed allison, no power mods. We have put over 50k miles on it in just over a year. About 30k of those miles are towing an 18 foot trailer. On the highway we get about 16 mpg loaded, no trailer 18-19mpg. The truck runs strong, pulls up every hill without a glitch and generally without downshifting. I do love the transmission, I think it makes the truck. Really nice ride, quiet inside and out, no turbo lag, glow rods rather than glow plugs (rods are larger than the plugs and heat up and ready to start really fast,) rarely see smoke puffing out of the pipe. Basic interior not many frills, but it gets the job done.
My dad has a 03' Ford F-350 6.0 liter, no mods. This is a tough truck, but not the most efficient. It doesn't matter if we have a camper in the bed with a 20' loaded snowmobile trailer behind and it gets 13-14mpg which is the same if you have no load. Its bizarre and kind of frustrating. Its only a 4 speed and thats probably why mileage isn't better. It tows well and handles the load well, but there is alot of turbo lag off the line. Overall a good truck on the highway, but not my idea of a daily driver. Rough ride, noisier than the duramax, glow plugs sometimes take a while to heat up in the cold, puffs some black smoke up the hills when it downshifts. Nicer interior and controls, more luxurious feel, comfort, and more bells and whistles.
If I were to choose one for myself I would go with the duramax. The duramax has better fuel economy with and without a load, no turbo lag, better ride and I think it turns a little tighter (both trucks are 4 door long bed)
I don't know much about the dodge, but I hear they get 20+ mpg, but thats coming from a dodge guy and we all know how they are and how much they can exagerate. Frequently I hear: "DODGE IS THE BEST" or "YA SHOULDA BOUGHT A DODGE" thats all a LOAD OF CRAP! Just the reputation and how people talk great things about them is enough to keep me away. I know one thing, I dont want turn into some hick dodge guy with an ego the size of china.
The bottome line: I am sure that you would be fine with any of the trucks (Chevy, Ford or Dodge) you buy as long as its diesel! Good luck truck hunting!