3500 single axle or 2500? Any difference?

I used to run a 2005 2500HD regular cab with a steel dump insert. The 2500 does have the smaller 10.5" ring gear but is still rated at 10K by the axle manufacturer. I used Timbrens to level the truck. At the time the limiting factor was the tires and wheels. With the 1000 lb. insert installed I could carry 2800 in the bed without exceeding any ratings. On a few short hauls it handled over 4K without issues. If I had wanted to carry more weight on a regular basis I could have swapped over to 19.5" Rickson wheels and tires. As others have mentioned, drivetrain components are the same. So for your application I'd say to get a 3500 if you can, but with just a few mods a 2500HD will do the same job.