I'm relying on my former Neighbors experiences and a few trips in air bag equipped F350s.
The air bags were used in towing a three axle Fifth wheel.
The F350 and especially the GM Chevy One tons are very firm already.
Extra Springs can be configured to come into play only when the load compresses the original pack a good ways.
The air bags can be inflated or deflated, and the Fifth Wheel rides level, and not as much extra pounding on passengers in the Truck.
Ride quality is a lot better with bags, from what I've experienced and testimony, of Friends who have went both ways in the past.