I have done both, 96 SRW, 03 SRW, 05 DRW, 11 SRW. It really depends on your needs. I found the dually to tow, and drive very nicely, but damn did I get stuck a lot in snow, etc. I did some towing like you are referring to with my 03, which was definitely over the limit, but it was almost entirely minimally traveled roads, in ranch country. I did it a couple times, and when I upgraded you can see from above that I got the dually. That was great, except for the being stuck part, and replacing tires. I am back to SRW now, and will definitely stay there, but if I were doing 20k miles a year or better in front of a 20k trailer I would get the dually again.
Still would stick with a manual though, but I doubt they consider that tow capacity in the cards for it. I do find it a bit silly so much of tow capacity depends on gear ratios, etc. Any of these trucks will get WAAAAYYYY more moving than they can stop. Seems it should be more on the chassis than the powertrain, but hey, I am just a dumb engineer.