I can comment on this pretty well as a long time diesel truck enthusiast. I currently own a 2001 dodge ram 2500 4x4, 5.9 Cummins, auto trans.
Here is what I would give as my opinion. Cummins is the best diesel engine of the "Big 3" trucks hands down. Nothing beats them for power, durability, ease of maintenance, operation cost, fuel mileage, etc. Not that the Duramax or Powerstroke(including the later 6.0s--stay away from the 04-05 ones though) are bad engines. Cummins is just better. However, Dodge does have issues to be aware of. If you are not worried about the year of the truck, a "2nd gen" RAM from 94 - early 1998 with the 12 valve engine is about bullet proof. The dodge auto trans until 2008 model year is not good in factory form, though. I have rebuilt mine with full billet parts and it is now awesome, but that was a $4000 undertaking a few years ago. It will last for another 150K miles towing heavy now, though. Be aware of this. The 1998.5 model year Dodge switched to the 24 valve engine, which had some problems until late 1999 with weak block castings. These early 24 valve engines are called "53 blocks" due to the number on their casting and are not good. Some last a long time, some crack in the outer passenger side web and leak coolant. PITA to fix with a metal stitch kit. Also, 2001.5(my truck build time) went to 4 wheel disc brakes which makes stopping with a load a lot better than before. If you go dodge, I would go with a 2003 - 2007 model, quad cab, and, if you are willing to shift gears, the 6 speed manual. GREAT towing truck. All this being said, here are my takes on the other options for diesel tow. GM has the best auto trans hands down in the Allison. However, not all duramaxes had this trans until the 2006 model year timeframe. Be aware of that as some sellers falsely advertise and/or do not know. If it has 5 speeds, it is an early Allison(great trans), if it has 6 speeds, it is a later one(even better), if it has 4 speeds, it is NOT an Allison, no matter how much the owner tells you it is! Though I have owned a lot of Fords over time(First car was a mustang, have had several of them since as well as a bunch of Rangers, and currently have a Volvo...sort of a Ford), I think the GM trucks ride a little better and are nicer inside. GM has independent front suspension in 4x4 so it is smoother. Dodge goes with solid axle, as do many Fords. Ford trucks do have great build quality and interiors, and are probably the best for overall cab durability though I consider neither their engine or transmissions to be the best. 7.3 is probably their best diesel engine.
I will caveat this following statement with my credentials: I am a mechanical engineer, automotive specialty, and did graduate work in diesel emissions control stuff. This being said, I would stay away from the current "emissions diesels (model year 2007.5 and later) that have diesel particulate filters (DPFs) unless you are buying a new truck with new truck warranty. They are getting better, but it is a technology that is not a good long term solution and controlling the injection strategy to make them work and also regenerate them without creating other problems with turbochargers and other equipment remains a challenge with all the makers. Also, diesels are so powerful and you may want to hot rod the truck as I have done with mine, and you really cannot do this reliably with the emissions trucks. I know what I am talking about with this. Not a slam on anyone who has done it, just want to give you good advice.
Bottom line if I were you, for a tow vehicle, I would look for a 2003 - 2007 model year(no emissions equipment), manual trans for dodge or allison auto for GM, 4 door cab, 4 wheel drive(these engines are so heavy you need 4x4 if you ever go off road, even to a wet grassy field). Also, go with 3/4 ton (2500) as it will ride a lot better unloaded than a 1 ton.
I am happy to answer any other questions. Good luck.