short verson: cummins, and if you go older, try to go with manual tranny, or plan on having the tranny upgraded.
Long version: see below:
I had a 96 cummins A/T, with 220K miles, ~20 MPG using odometer and fuel pump, then graduated to a 99 cummins A/T, with little edge box, ~23 MPG, and more balls than you need. Went to a Duramax/ Allison for larger cab size and from day one, wish I never sold the 99 cummins. that Duramax is a gutless wonder. The first time i towed an empty trailer with it, I thought the trailer brakes were stuck! I had to put an $800 edge/Attitude in it just to get it to keep up with the Cummins I had, then, dial it down to milage setting to maybe get 18. Finally got fed up with it, went back to 2006 Cummins, A/T, (mega cab) very happy, 25 highway stock, 20 in stop and go Seattle Traffic. 18 loaded with sled deck, 2 sleds, while driving up 2-lane winding mountain roads, driving it like I stole it. (the GMC was lucky to get 10 in same situation) Its your money, but for mine, I won't leave cummins again.
At the shop I used to work for, we would draw straws to see who had to work on the Ford products. The owner always hated the dodge products, and loved GM. When I started my quest for a diesel truck, I ended up with that 96 dodge, and what a great truck. I dont consider myself brand loyal, but I am product oriented. Produce a superior product, and I'll buy it. I think the half-ton chevy gassers are great, but for some reason, they couldn't get it right with the Duramax. The Allision is awesome, wish I had it behind the cummins. Also, that 3/4 ton diesel did really poorly in the mud and snow. (I've been in it plenty to know when it's the truck, and when it's not)
Feel free to PM me if you want to discuss more.