If you're going to be towing full-time, diesel. The maintenance will be significantly more over time as the Emissions equipment on them will fail and cost a lot to fix.
What maintenance is significantly more aside from the chance of an emissions failure?
I've posted the numbers before disproving this myth, but I'll give the abridged version.
- Oil changes cost more, but are required half as often as a gasser.
Average cost to get an oil change on a 6.4 Hemi here is $112. Ram says change it every 8k miles.
Average cost to get an oil change (my last one) on a 6.7 Cummins here is $154. Cummins says change it every 15k miles.
- Fuel filters are required every other oil change, but there are no plugs or coils to contend with, so that's really a wash.
If you swap all 16 spark plugs and 8 coils at 100k miles, the OEM parts will cost roughly $871.
If you change both fuel filters with genuine Mopar/FG, by 100k miles you'd be at 6 changes and the cost would be roughly $850. It's a wash.
Everything else is specific to the truck and not the engine so there are no deltas in the cost.
The "only" maintenance cost delta is the emissions equipment and there are two options. 1) Replace the bad component or 2) delete it if you can. If you use the truck to tow a lot and the emissions system gets a good workout there's a likelihood that you'll go a long time before needing to replace it.
Before we get into fuel costs.... that is not as far off as some think. It depends on how you use the truck and the specific set up.
For towing you'll get much better MPG than a gasser but you may suffer with the unloaded diesel mileage depending on your gears. That said... I got 18.5 mpg on my 5.7 Hemi on the interstate with 3.21 gears and the ZF8, and I get 18.5 mpg mixed in my 6.7 with 3.47 gears behind the 68RFE. I get closer to 22 mpg on the interstate (empty).