While I don't actually own a 2500 I have driven and towed with a 2017 Ram 2500 w/ Cummins to the tune of about 1,000 miles and I also did test drive a 2021 2500 with the 6.4. Although they have coil in the rear, I still find them to be a bit rough riding for my old man self. I also feel like I sort of wrestle them down the road with the steering feeling heavier than I'd like for a long drive. They have a very truck like feel to them which can be both good and bad I suppose. I do like the ride height as I feel like I sit up much taller and feel almost like I have a more commanding presence when tooling down the road. The 6.4 with the 8 speed is a great combo and really scoots it down the road - effortlessly. When test driving the '21 it seemed (to me anyway) to roll and almost push through the corners more than I liked. The 6.4 2500 road rougher than the Cummins obviously due to the weight of the diesel but was very much quieter. At highway speeds it was very smooth overall but continued to let you know you were in a 2500 at every bump.
From what I can tell, I believe the 6.4 2500 will get approx 3 - 4 mpg less than a 5.7 1500. I originally considered the 2500 over the 1500 simply due to the price. One can get a well optioned 2500 Tradesman 4x4 crew for very very close to a decently equipped 1500 Bighorn cc, 4x. All in all I think it comes down to your specific needs. If you're going to be towing 8,000 lbs or more on a constant basis or if you need the payload (6.4 2500 cc 4x has close to a 3k payload) then the choice is clear. If you enjoy quiet, effortless steering, comfort, good handling, better mpg, under 7k towing, and the ability to wash it without a ladder, the 1500 is the winner. Either way they are both hella good trucks.
I should add that I did use my iphone to record decibels in the '21 2500 with the 6.4 and it came in at 64 decibels at 65 mph on a newly paved dry concrete freeway. My wifes '17 Rav4 comes in at 66 on same highway and my '21 1500 cc, 4x, 6' 4" bed came in at 57 decibels.
