well after reading your first post you plan to load it up with 4000lbs of toys? it will tow that camper unloaded with no legal issues. maybe you should only put in about 2000lbs of toys

 

One other difference no one has mentioned. The 3500 has a Dana 80 axle, vs. a 70. Besides Dually's ,the axle, I believe, is rated for heavier loads. Also, the cooling systems may be different, perhaps, seems like they had a lighter duty for some 2500's.
Maybe some other differences?