How much do you want to spend? That will decide what you are going to get.
If you are trying to stay below 25k then an older Ford 7.3 will be your best bet. Good looking truck and an engine that is tough as nails.
If you have a little extra dough I would get a newer 04+ Dodge with an auto or a Duramax. They are definitely better than the 03.5+ 6.0 Powerstroke.
Dodge cleared up all issues with their autos by 04, they are very stout trannys now and do not have issues like in the past.
I recently bought a 05 1/2 ton ram with the 5.7 gas and I wish I would have spent an extra 5k and got the 3/4 diesel. Still happy with my truck but after riding in a buddies hopped up diesel last weekend it made me want one. Must be something about 760lb/ft tq at the wheels.
What exactly are you looking to tow though? 1/2 ton tow capacities are pretty impressive now. I wouldnt want to haul a huge enclosed car trailer but I plan on towing my friends Talon to the track a few times this spring.