At one time, the manual was rated some 4-5K lb MORE than the auto was, at least from Dodge any how. GM the muncie and NV tranny's were rated the same, except initially the NV, same as dodge used was down graded by 2K lbs. YET< the manual behind the 6.5td had a better overall low, better initial starting, less overheating issues, was faster up grades with equal loads......still wish I had my manual 6.5 at times! later years the manual was rated the same in diesels as it was in the gas rigs.
Ford I am not as knowledgeable. but I am generally recalling similar gcwr's in the auto vs manual.
Reality is, with tow ratings, to some degree, it is a what they want to sell more of. For the future, it will be auto tranny's. So they will get higher ratings, EVEN IF, they are the worst overall one to tow with! from a performance standpoint. What the auto engineers are able to do with auto's, since they can control things electronically, they can tweak how the torque or hp is given to the rear wheels, so today any how the autos can be a better setup, more torque and hp!
A bus driver for the ski school I work for used to use his foot for controlling the bus going up the hills. He found out if he used cruise, the computer gave him an extra 20hp! so he went a bit faster in cruise than he did with out!
Which is truly better?!?!?! I figured out at one time a manual dmax with 540 lb ft of torque would go up a 40% grade in first before literally stalling out due to lack of power. The auto stalled on a 24ish percent grade. Even when the dmax had 650 lb ft like mine, I still do not keep up with a manual with less torque from an initial take off on a steep grade! The auto also uses an additional 40 hp or so just to run the that system. Ie more drivetrain loss with the auto. So the manual dmax actually has better performance overall!
BUT, from a 2500 vs 3500 standpoint from which will tow more. The one with the most payload will always have more potential to tow a higher lb trailer! Then the one with the most RA capacity, will also have the best ability to tote a higher trailer. A rule of thumb I learned years ago before gcwr's became the norm, was 2x the grawr for the max trailer behind a given rig! I find that works, as when I have gotten into the 2.5 x range with some of my rigs in the past, the trailer DOES wag the truck a lot more! so if I stay under the 2x, I have a much better towing experience, EVEN IF the truck has the gcwr/power to handle the trailer, at some point in time, the trailer wags the truck!
If you want to pull a16K trailer with a 7K ra rig, yes it will work, but an 8500-9K ra of the 3500 will be better overall. That is not to say as shown by some, one can't pull a larger trailer than my ratio......there are a few other rule of thumb tow ratings one can work with, again not etched in stone, but if you follow them, it is a better way to go.
Marty