We have a GMC 2500 Duramax w/Allison, crewcab with the 6.5' bed. We tow our boat (C-Dory 25) which weights less than an R-31 and a HitchHiker 5th wheel, which weighs more than an R-31. Over 40,000 miles towing the 5th wheel, and zero issues with the short bed and a non-sliding hitch. As the selling dealer told me: if you need to turn MORE THAN 90 DEGREES, you are putting WAY too much torque on the trailer axles/tires.
For heavy bed loads, the dually is a good way to go. For pulling a boat out of a slippery ramp, you will appreciate single rear wheels and 4-wheel drive. Our truck is single rear wheels and 4WD, and does great with the boat and the 5th wheel. If you go with a frame mounted hitch, I'd suggest a weight distributing hitch for the load weight of the R-31. A gooseneck would be a good way to tow that load, since your truck is probably rated for more towing weight with a 5th wheel/gooseneck arrangement (ours is).
From my perspective, the 3500 can handle more bed weight; the trade off is a rougher ride when you aren't towing. Towing an R-31 is not something to take lightly - besides the BIG weight, the boat is a 10' beam, requiring permits in any state you travel in, and restrictions to what roads and when you can travel. MORE truck is always a good idea with heavy loads.
The truck is our daily driver most of the time. The crew cab w/ 6.5' bed can fit in almost any parking spot, but the turning cut of the front wheels means you will probably mkae more "maneuvers" getting into the spot than you would with a car. I usually look for a spot further out - saves on door dings and the walk is good for you. 😉
If I had the $$ to put into a 31, AND I was considering towing regularly, I would looking into a medium duty/business class truck... the right tool for the job.
Good luck with the decisions.
Jim B.