The manufacturer's GCWR figure is the important one as it covers the entire truck including the frame, suspension, engine, brakes, and drivetrain in terms of the maximum load it can handle. Weight in the truck counts as does the trailer's total load. The numbers tend to be conservative ones and whenever the truck companies increase the rating they have made modifications to the parts of the truck to take the added stress.
Your 1/2 ton will be lighter than the 2500 so the power to weight will be better. You may need to use a WD hitch with the 1500 but that should be the only change from what you have been doing with your 2500 truck.
I would go with the lowest gearing that Ram provides. With 6-speed transmissions you get two overdrive gears to cover freeway fuel economy when not towing and when towing the lower gears will shorten the time it takes to get to freeway speeds and safely merge with traffic.
