It depends on how old a truck your looking at.Since the 6.0 Liter PSD in 03,they used torqueshift 5 speed auto.The 03/04's had issues,that were resolved,they seem to be a solid trans now.The older 4 speed auto trucks,with the 7.3PSD,I wouldnt want one of them if you were going to run loaded all the time,transmissions are worked real hard,most of the 4 speed autos went out under 50K miles on 450/550s that plow/tow heavy.
If you are willing to waiver from Ford,dont count out the used C4500/5500 Chevy/GMC's when you search for a truck.They are solid,and have the allison auto behind the duramax diesel or 8100 big block,both solid powertrains.They have much better visability,turning radius,and are heavier duty than the Ford450/550.Transmissions are better,and IMO,so are the engines.