Well in my ever humble opinion, it's MOPAR or NOCAR!!!!!!!!!!!!!!!!
Seriously, the Cummins inline six is The Best diesel engine out there, as far as longevity goes. It is the only true medium duty diesel offered in a pickup truck. It is all iron. The GM is iron block with aluminum heads. I'm not sure on the Ford, but I believe it also has aluminum heads. Aluminum and iron expand at different rates and that causes trouble. If you read "Diesel Power" magazine, the ones that make the craziest horsepower that hold up are the Cummins engines.
Are the Dodge trucks as fancy as the Ford's? No.
In stock trim, is the Cummins as powerful as the new Duramax? No.
But, which one routinely goes to 500,000 miles before overhaul? It's not GM or Ford.
Go ahead and throw arrows as you please...but I see a LOT of Super Duties broken down along the highway with their hoods up....
Biggest problem with the Dodge's is that if you hump up the output of the engine (which it's very easy to make just ridiculous amounts of horsepower with....it is NOTHING to make 700hp and 1200 ft-lbs of torque with a Cummins) you will lunch the factory transmission. OK, so you send your busted tranny to ATS and give them $6K and they will send it back to you so that you will never break it again. But, the weak link in the Mopar drivetrain is the tranny. At least with the older ones. The new one is supposedly so similar to the Allison that Allison actually sued them (and lost) and the new tranny is supposed to be really stout. But, in modified form, the old one is run in many 10 second trucks.
Bottom line, they're all decent rigs. But, you won't have to apologize to anyone for running a Dodge Ram with a Cummins turbodiesel.
For what it's worth, I run a 2004 Ram with a 5.9 Cummins factory rated at 305hp and 555 ft-lbs of torque. I pull a 34' Avion triple axle that is actually 36' long and grosses right about 10,000 lbs. With my truck, I can light the tires up pulling away from a stoplight (with or without the trailer hitched up). I have not encountered a hill yet that I could not accelerate up while pulling this trailer, and that includes the Great Smokey Mountains down in TN.
I used to get 21mpg with this truck until they changed the diesel fuel formulation in 2007. Now I get about 19mpg empty, and around 12-14mpg towing. I've not run into a situation yet where I needed more power.
That being said, the new Duramax is 397hp and 765 ft-lbs from the factory. I could match that easily with a Banks system, that I may install since I just paid the truck off (six years financing....probably the last new new vehicle I ever buy). But, I am leery of blowing the transmission. I really don't need any more horsepower, but it'd be fun to be able to dust Camaro's stoplight to stoplight (that's what my '70 Charger with the built big block is for....).
Anyway, good luck in your search. From what I've seen, it's tough to find a reasonable mileage diesel on the used market. They're such nice trucks, folks tend not to sell them.
Ford IS making deals now on their new ones. And, that new F-450 is "almost" enough to make me leave Mopar. Almost
Get a Dodge and be happy!