tgreening
I have glanced at air bags, but Ive always wondered how durable they are. My truck is a few miles shy of 200,000, and a lot of those were hard, loaded down miles (my dad bought it new and always have weight in the back). The springs are not terrible, but they are definitely on their way out.
If you always have weight in the back, you should look more towards springs than air bags. Air bags are great for when you carry weight on and off. You can firm it up and get your ride height back, then bleed them down when empty so you're not bouncing around like crazy. Helper springs do the same sort of thing... they will help take the load (hence the name) when the springs are compressed, but are not part of the suspension when the springs are relaxed. Bigger/stiffer/more leaves in your spring pack will stiffen the ride all the time, and lift your truck when empty.