2019's

Can't comment on the Duramax - never owned one, but I towed 12k with a 6.4L Hemi and had no complaints. It usually ran around 8 - 9 mpg pulling the same 5er I did with the 5.9L Cummins which usually made 10 - 11 mpg, so +2 mpg for the diesel.

Interestingly, the 5.7L Hemi I have now makes 9 - 11 mpg pulling 3500# :). I thought it would do better.

I bought my current truck at the end of July just as the new models were coming out. And the price was $14,000 off sticker price trying to move them. That was price reduction plus rebates. Worth it to me to take the one model year old truck that looked exactly like the newest one.

Now if there had been a significant change between the two years then I might have reconsidered.

The RAM website allows my to build a 2019 model.

I have towed heavy with a 2015 3500 6.0 and with 2015 D/A and 2018 D/A. Towing about 30-31K combined up 6-7' mnt passes and living at 4700' elevation the 6.0 got 5-6 mpg towing that heavy. The Duramax trucks both older or newer are about 8-9 mpg. Obviously the Duramax is better towing experience, both up and down the hills (which you don't have). The mileage difference is not that great in my experience. Factor additional cost for Dmax and price increase for diesel over gas (at least here) it becomes a tougher decision. If you are pulling heavy a lot then the Dmax advantage really shows. If your pulling around 10K on flat ground, you have to knock down a lot of miles to get your money out of the Dmax.