Actually, it's both. The only physical difference is the one extra rear spring on the 3500 SRW and the spring and axle on the DRW. Those of you who don't have a 3rd Gen truck may not understand the differences in how the 2nd Gen and 3rd Gens handle the weight. My '02 2500 has carried far more weight than my '04. 5 and the '02 was stable. I would not put that much weight in the back of my '04. 5 simply because the rear end would sag so much and probably be hitting the bump stops. I would likely be legal doing so with my '04. 5 where I was no-where near legal in my '02.
That being said, I would advise your customer to get the 3500 DRW version. It would be much more stable for the camper and towing/hauling a heavy load in the bed. Your customer would also likely be installing air bags or something else to level the ride.
Your customer would also be able to carry more of a load legally if they went with a 3500 DRW.
These are only my thoughts and opinions. Take them as they are and remember you get what you paid for.