It used to be a simple decision... get the 1 ton because they both ride the same empty and the 1 ton can carry more weight.
Ram made the decision more difficult with coil suspension on the 2500, making a softer and smoother ride than using leaf springs.
That being said, with the heavy Cummins and the prospect of towing a 5er, I would STRONGLY encourage you to look at the 1 ton trucks. If you can afford the factory air option, I would tell you to take a back-to-back test drive with the leaf and air ride on the same route and compare the empty ride.