..Think we've had this discussion before:h
Don't know about the manual dually, I do know that my dually has a 14,000# GVWR and 9750# weighted rear axle. With the 4:10 GCWR is "up there":)
I towed the 5th wheel in my sig. with a 2010 Dodge Dually, same configuration as in sig, but Laramie , but 68RFE tranny, 5er weighed 15,650# or so, pin weight around 3200#, no problems whatsoever..Went to the 12' with same amount of HP, but 800ftlb of TQ, did even better and 5er weighed around 15,850# with 3300# pin weight (washer/dryer combo added)...NOW, I'm just waiting to get an adapter, or different hitch to hook this beast up with 385HP, 850ftlb of TQ and Aisin tranny to see how it does AND what kind of mpg I'll get...Love this truck.
Don't think that 3500SRW has "my" truck beat in payload, I know it doesn't in GVWR(14,000#) or GCWR, or rear axle weight rating(9750#)...the dually offers a more stable towing experience on mountainous, curvy roads.
The one comparison the OP used, is probably the only one that would work in the 3500 SRW trucks favor, even the 3500 Dually auto with 68RFE tranny will have more payload and tow more....it is what it is.