[Gary-Alberta]

Using a GMC 2500 tow vehicle, I researched both, adding extra leaf spring each side, and adding air bags. In our tiny Canadian dollars, springs were $800, bags $1400. I did both.
The springs, obviously, increase the capacity of the suspension. The springs worked so well the bags really were not necessary.
You have to decide whether you believe that the modification increases the truck capacity. I have modified many vehicles in my time, and I believe that modifications DO change the vehicle. Many others do not agree.