I had a 2005 2500 Cummins Quad Cab with the 4 spd auto and 4:10 gears. I traded it in on a 2016 2500 Cummins. The new one is a mega cab with the 6'4" bed. It has a 6spd tranny with 3:42 gears. The new 2500's also have rear coil springs in place of leafs and mine has a rear anti-sway bar behind the rear differential. The 3500's still have leafs. Both were/are 4x4. The new one is long, and you can tell when trying to park.
Old one got around 14 mpg in town and while towing normal trailers at 70mph and 10 mpg pulling our 5th wheel. It would get 18 mpg unloaded on highway. New one gets around 15 mpg in town and around 21 highway (empty). Haven't pulled 5th wheel far enough to determine highway mpg. All of those figures were hand calculated, not computer.
New truck seems slower off of the line. Old one could smoke the tires, new one not so much. But, mid-range and passing is better with the new one.
The ride with the new one actually feels stiffer on rough (washboarded roads). But, I thinks it's just a perception.
With the 6 speed tranny, it really cruises nice at highway speed. Pulling the 5th wheel, it does very well. In fact, it does great.
Comfort-wise, I sprung for the Longhorn edition. So it has all of the bells and whistles. The seats are very, very comfortable. More so than anything else I've been in. The teenage boys and the labrador love the mega cab, especially after being crammed in the back of the quad cab. The rear camera and the bed camera are very nice when hooking up trailers. I didn't think I'd care about the bluetooth, but have to admit that I love it now.
