I'm going to go a different way. If we were comparing the 5.9 Cummins, yes. The 6.7L is great, but still has all the extra external failure points, which is what will fail on either one. A properly bulletproofed 6.4L with half the mileage for the same price range? Further evidence of the Cummins fanboi tax. Lots of people LOVE their Cummins, and there is a ton of great stuff about them.
Either one will have three times the torque you need to tow
Either one will be better than gas MPG
IMO, the dodge truck that was built around the Cummins is a nightmare. Terrible materials and build quality, quicker to rust, and don't even get me started on the 68RFE transmission. When it goes, it won't need a rebuild because the clutches wore out, it will need sprags, a new valve body, and you'll be very lucky if the valve body hasn't caused a crack in the case. $4000 MINIMUM for a build on someone's bench. The fact that this one hasn't needed to be remaned twice is a miracle, or maybe it has and the seller isn't disclosing it. Dodge took a rattly inline 6 diesel and put a transmission behind it that is basically a 727/A518 that has been continually band-aided since 1985. The 5R110 in the Ford, however, has line pressures that tend to be lower than some tow-folk like, which is easily fixed with a $60 kit and an hour. My years running transmission repair shops has me a little on a soapbox about transmissions... things that many people overlook.
Familiarize yourself with B10 and B50 life expectancy. They are mileage numbers that correspond to relative lifespan at which 10% and 50% of diesel engines require a major overhaul. These are not subjective, they are reported data from millions of engines. The B50 lifespan (that is to say that 50% of a certain engine in a production has needed a major overhaul) for the 6.4L Stroke is 375,000. The B50 life of a 6.7L Cummins is 250,000. That Cummins is fast approaching almost double of its life expectancy based on the average. Buying that Cummins is like adopting a 17-year-old dog with cancer and one leg.
The 6.4L however is tucked inside one of the best-built, better quality, better assembly vehicles I have ever owned. When I would go from my F250 to a 6.7L Ram at work, it was like going from an Escalade to a 1975 IH Scout. Buzzes, rattles, squeaks, vibrations, punishing ride, torn seats at 80k, and good lord you practically had to take off your seatbelt to reach the radio or the cup holders. They have become much better in the later years, but still they're a big plastic rattlebox.
I realize that truck and diesel opinions are like buttholes. Every one has one and they all stink. I'm not brand loyal by any means. I've owned them all, and worked for a utility company in maintenance where we had four brands of truck, gas, and diesel. The only ones I've consistently regretted were the Rams.
If you find a good Cummins and you like it, go for it. In all honesty, maybe you're the type of person who prefers max towing capacity and you'll sacrifice some other amenities. No judgement. I'm speaking for myself here. Just please don't drink the Cummins Kool Aid. You'll overpay and get a great engine with a glass transmission and less "truck" for your money. Even the venerable 5.9L was wonderful, but the fanbois made them so expensive that it wasn't worth the buy-in back in the 90s and 2000s