We run Cummins cause they last forever, a place in Fort Madison had one with 500,000 on it, the sold it to another guy and hes got it up to 800,000 now,
Personally the most we ever put on one was 300,000 on a 95, but it ran like a top and had more power than the 98 we bought when we sold it
The only problem dodge has is after a few years the electronics start going bad: radio, fuel gauge, headlight switch, etc. Not a big deal if you got a salvage yard close by, sometimes kinda pricey if you go with new
We've never had a duramax but i have talked to some people that do, so far the only guy i have heard complain said sometimes all of the sudden his get so you couldn't run over 1800 rpm, he said it usually didn't last long but it was a hassle, he also said it burned up injectors pretty quick, i think the reason he was having so much trouble is that he put a chip in the truck to get it to turn (according to him) 90 more horse than factory, which is a hell of a lot to turn an engine up, he said the last truck he had was a cummins, and he put a chip in it and kept turning the center out of the clutch
As far as the powerstroke, i know the 7.3 model was expensive as hell to work on ($3000 for injectors) and according to the diesel mechanic we go to they were his best customers, haven't heard much about the 6.0
for gas mileage our cummins (not the 24 valve so i don't know how valuable this will be) gets around 15 without a trailer and about 12 or a little less with one (i can't remember for sure) but it is very low geared
another note a guy we know that also does custom trucking has a 7.3 powerstroke and he is on his 3rd or 4th transmission, he has also tore the rear end out of it a time or two, dodge did have some transmission trouble around 97 but i think they have it all worked out now