OK, as previously posted, I own an '01 F350 w/PowerStroke diesel and my dad owns an '01 Chevy 2500HD w/Duramax diesel. Both are 4x4s and both have auto trannys.
We were towing a couple of trailers recently, and decided to run a comparison. The trailers were 8400lbs loaded and were "tag-a-long" (Reese hitch) type.
After six or so miles, we switched and finished the drive home (another seven miles).
Head to head, the Chevy simply pulverizes the Ford in towing power. It stays in a workable part of the powerband most of the time with the 5-spd Allison automatic. The three speeds that the Ford works with in the towing mode leave the tranny pulling in too-high of a gear if going up hills.
In virtually every other category (ride comfort, visibility, stability, etc.) the Ford takes the edge.
Dad said he preferred to tow with the Ford but liked the drivetrain of the Chevy. His trailers can weigh up to 10,000lbs so he needs more power than I do. My one trailer weighs 4,200lbs loaded so balls-to-the-wall power isn't as important to me.
Neither one of us liked towing with the Chevy as much as with the Ford.