I have had 3 dodges, and a GMC. My first diesel was a 96 Dodge CTD, AT. 190K when I bought it, 220K when I sold it. Great truck. Trans never failed, me, but I knew they were not that great, so I was nice to it. Totally stock.
Second diesel was a '99 Dodge CTD/AT. That was a monster. It had a Edge box, and some tranny mods to it. Loved that truck. Then I got married, and we decided we wanted a bigger cab. The Megas were new then, so we went with a .....
Third diesel was an '04 GMC Duramax/Allison. What a turd. Absolutely gutless as all-get-out,-but at least is got horrid mileage!
I had to put an Edge Juice/Attitude controller in it just to get it to try to keep up with my previous 99 Dodge/CTD. Also, they had a recall on the fuel injectors, but they wouldn't replace them until they cracked, and filled the crankcase with diesel. (and they said they would not cover the bearing damage from diesel in the oil, or the towing). It is possible we had a lemon, as there are lots of folks that love them. We HATED ours.
Finally got so fed up with it, we sold it and went back to Dodge. The Megas were available used by then, and I found an '06 Mega, CTD/AT. Great truck.
In my OPINION, the Dodge design is substantially more robust than the GM. The stock GM rode better than the stock dodge, sure, but the drivetrain is so much stronger on the Dodge- it HAS to be, since that Cummins is so much more powerful (torque) than the other designs. Ford may make great chassis, (not sure on this one) but they suck an making a diesel with any durability since the 7.3. (actually IH makes the Powerstroke for Ford) Anyway, The 6.0 can't keep head gaskets around longer than 100K, has a problem with the EGT design. The Duramax gets crappy mileage and has transmission output shaft housing issues (they are prone to cracking) and the Dodge has trackbar design problems. (PM me for my chosen solution) They all can benefit from better Torque Converters. All todays diesel engines can push past the limits of the the AT's the put in pickup trucks.
The inline-6 design gives inherently more torque. It's a math thing. Same displacement, fuel, etc, = more torque. In the larger diesels, Cummins is NOT the best, but that darn B-series just flat works.
Vinnee is right, the 'robustness' (like my new word?) of the Dodge drivetrain dwarfs the other two. It also still uses a solid front axle (strength) with coil springs and trailing arms (ride and strength).
I don't mind telling you I'm biased. I am. But, I too have worked as a professional automobile mechanic, and I have a bachelor's degree in vehicle design. I'll take the Dodge with the Cummin's engine. The reason they went to the 6.7 was not about power, it was about emissions. They can 'detune' the 6.7 to 'meet the emissions' and the couldn't do that with the 5.9.
And, I'm not trying to bite anyone's head off. I promise. I just want to share with you what my personal experience was, and what my education taught me. I think the GM's look the best, I like their interior ergonomics too. Ford makes pretty good frames, and their stock AT is pretty solid. The Dodges tend to pull to the right; I believe there is a offset ball joint kit you can use to solve that. (which by the way, I think is absolutely crap that you should have to put an offset ball joint in to keep a $45K truck from pulling to the right!) but, it's cheaper to fix than fixing the head gaskets on a Ford, or adding propane and twins to the Duramax.
OK, there you have it, my two cents.
good luck!
PE
