There are two different things, which it is best not to confuse.
Air assist is little airbags that replace the bump stops and leave the springs doing the work. I've no experience of these, but I guess they are stiffening the springs and may make things better.
Air suspension is where the springs are removed and replaced with air springs. This costs a fair bit more but is truly wonderful. You get a smoother, quieter ride, much less sway and of course and even ride height whatever the load.. The only downside is that after a few days parked up, you have to wait three to five minutes for the air pump to refill the air tank.
Oh, and that there is much more to go wrong. On mine, over a period of about fifteen years, I have replaced the pump twice, the air springs twice and the air tank once. Only the pump was expensive. And I've also replaced the long parabolic "springs" twice. They're not expensive, but have to be custom forged, so the van is off the road for a few weeks whilst they make them.
As it happens, I travelled on our local village bus service this afternoon. It's a fairly new bus, based on a Mercedes Sprinter with a nice smooth, powerful engine, and the ride near the front is fine, but sit near the back and its an entirely different story.
Every bump or pothole smashes through the frame of the bus. Lights are rattled off the ceiling.
It's awful. The drivers bemoan the cheapskate approach of not specifying air suspension,
