I have the Firestone Airbags, and the difference between 5 psi and 100 psi in the bags is about 4.25 inches in rear lift. My leafs are tired, and I carry a couple hundred extra pounds than if the Van were empty. I put 10 to 15 psi on the passenger side bag, 15 to 25 psi driver's side to level it out. They really help minimise how much a tractor trailer pushes me around at highway speeds, and significantly reduce lean in turns. The higher the PSi the less lean.
Airbags must have at least 5 psi to prevent chafing, and will effect the ride somewhat depending on vehicle.
Timbrens, depending on application might not engage until the suspension is compressed a certain amount. So Unloaded it might ride exactly the same, where 5 psi in the bags might make a noticable difference, or not.
The Adjustability of the Airbags is the biggest bonus over Timbrens.
The biggest advantage of Timbrens is they are an easy install and forget system, no worrying about PSI changes, or bag puncture, or possible having to modify the exhaust slightly to keep it a few inches from the airbags.
There are other products like Shocks with coil over springs on them, and extra helper leafs which clamp on the leaf springs too.
Less palatable products in my opinion. I don't believe the shock towers/ mounts on most vehicles were designed to lift the vehicle weight, and you might not even have rear leafs to clamp a helper leaf to.
Note that adding Helpers does not increase the overall amount the vehicle can carry or tow, it just makes the rear sag less.
Consider adding a transmission cooler.
