I'm looking to sell by 1985 Ford F250 4x4 gas powered truck and to upgrade to a diesel. I pull a 20 ft. aluminum gooseneck horse trailer and live in the mountains. Can't afford a new truck, but am considering any of these:

2003 Ford F250 7.3 diesel
2007 or newer Chevy Silverado or GMC Sierra Duramax w/Allison tranny and towing package.

I've never ridden in a Dodge truck that doesn't ride like a truck, so am not really considering one at this time.

Any recommendations, warnings, comments?

Although my 1985 truck probably qualifies for the "Cash for Clunkers" program, apparently none of the diesel trucks MPG has been rated by the gov, so the truck at the buying end has been left out of the equation.

Thanks,

Any of the 7.3 PSD Fords have been reliable. However the transmissions are not up to the standards of today's modern ones. So It will likely need freshening up if you get one.
The Duramax/Allison combos were for the most part pretty good. The LB7 did have problems with injectors, but they are warranted for 100 or 200K miles. The LLY from mid 04 to early 06 has had it's share of overheating problems. Mostly in the 04/05 model years. There is a fix for the problem that will only cost you a couple of hundred dollars. But not from GM. They abandoned the owners of those years rather quickly.
The later Dodges are probably the best of the best. The Cummins coupled with the 5 or 6 speed manual is one stout mother. I doubt you could wear it out or break it unless you abused it to death. The newer versions are much smoother than the 90's versions. And can be bought for probably the same price as the 7.3L Fords Remember the 7.3 is very old technology compared to the Duramax or the later Cummins.