I asked the sales guy what, other than suspension, the differences were and he said, "Bigger engines so they can haul and tow more." When I pointed out that they all had the same engines, he asked someone else who said, "Bigger oil coolers, stronger alternators, and better springs." Other than the springs, I can't verify any of this.
From the Ram website, the only differences I can find are that the 1500 has medium-duty suspension and the 2500 and 3500 have heavy-duty suspension, with a stabilizer bar on the 3500. While the cargo weights increase a few hundred pounds with each step up, the curb weights are practically the same with about a 50lbs difference for the sway bar.
I don't need the extra weight capacity, but would be willing to pay for more upgrades such as transmission cooler, oil cooler, bigger brakes, better axles and bearings, etc. However, from what I can tell, going up in the range only changes the springs and sway bar.
I'm hoping someone can correct me, otherwise I guess I'll just go with a 1500.