Ford Powerstroke. Done.
I searched for almost 2 years to find one. Finally got one. 99 Stroke, 98k miles, short box.
You can get cheap Chevy diesel 6.5L vans, but you won't really have much power/torque benefit over a 6.0L gas. You will save money on fuel since the MPG difference is more than the diesel cost increase in most states.
Dodge doesn't offer a diesel, nor do they offer a transmission that will take the punishment. If you get a dodge, drill out the check valve in the cooler line, install and aftermarket cooler, and cross your fingers.
Chevy has some nice ergonomic things, but overall the engineering is not what I'd call good. I was in charge of maintenance on a fleet of 136 vans and trucks for a utility company. The overwhelming complaint on our GM vans was that the A/C kept quitting. The condensers are ridiculously inefficient and on hot days the A/C would overheat and kick off the compressor.
Ford Pros: killer A/C, bulletproof E4OD trans (up through 99), shorter wheelbase/turning radius.
Ford cons: ridiculously tight engine bay
Chevy Pros: 6.5L reliability (when you move the PMD to a cool location), taller box, 4L80E cheap to fix when it dies (which it will)
Chevy Cons: Inefficient A/C, turning radius, Duramax commands top dollar, but no power benefit since it still comes with a 4L80E.
Dodge Pros: Um.... can't think of any.
Dodge Cons: Um... don't get me started.
Edit.... thought I'd add this. A week after I bought my 99 'stroke E350, I hitched up 12,000 lbs and towed 3500 miles round trip in 6 days. A week after that I towed 10,000 lbs for 1500 miles in 2 days. Every time I stopped and checked the tranny fluid, it was mildly warm to the touch. I averaged about 10 mpg with 12k lbs of trailer (and a loaded van), and I'm getting 18-22mpg empty.