The real world answer is that stock to stock a new SRW 3500 is only rated to carry about 500 lbs more weight on the rear axle. That's easily remedied with aftermarket options if need be, effectively creating the same truck. The one thing you will exceed on the 2500 as you approach the RAWR is the class 2 GVWR limit of 10k lbs, but that's an on paper limitation only.
If the paper limitation concerns you or is more important to you than the difference you notice in ride, go with the 3500. If it isn't either will do the job. This assumes you are looking at the current models and not older used options.