A GMC (or Chevy) would normally have more payload than a RAM. You would have to look on the door sticker of the individual truck to know for sure. A 2500 GM truck will probably have the nicest ride and most car like handling. GM uses an independent front suspension on their 2500 and 3500 trucks where Ford and Dodge use a solid front axle. The IFS makes for a much better ride.
Go drive a 4 door short bed 2500 and see how you like it. It doesn't cost anything to drive one. If you really want to keep with an SUV you could look for a 2500 Suburban; their payload is in the 2,000 pound range. They stopped making them after 2013 so you would be looking at an older vehicle by now. I really like my 2009 2500 Suburban. I can put 9 people in it with 3 on each row. That load, along with coolers and gear in the back and then 6,500 pounds of boat and trailer is no problem for it.