2500HD duramax. What can it handle?

Seeing you have already invested good money into a diesel, I would be looking at a lighter 5R. There are many lighter units out there, and because of the number of slides that are now available on todays unites, larger longer unites are not as important as they might of once been.
Opposing slides can make a smaller unit feel much larger than it actually is. The exception of this is if you have a large family with you, then it just makes sense for a larger heavier unit.