When I bought my gmc truck last year I was looking at that same decision, I resurched as much info as I could find on line and asked when I saw someone with one of the two, here's what I came up with.
Motor,tranny,brakes are the same between the 2500hd and 3500hd srw, you get an extra payload cap with the 3500hd srw because of the overload springs,bump stops.
I even asked at an used car lot the resale value between the two and was informed by the lot owner that the 2500 holds it's value better than the 3500 srw because more people look for the 3/4 vs the 1 ton srw.
I ended up with the 2500 cause its rating where way within of my needs for my 5th wheel.
Hope this helps.