I assume you mean settles.
Since it's for load handling, I'd say airbags. Airbags have a much higher load capacity then springs of similar size, so you would be able to lift the rear up and regain some suspension movement, at the cost of ride harshness of course.
The airbags require more work to install, more equipment, and more upkeep then helper springs, but that's the way I'd go with load control. Either would work just fine, but with the 'bags, you'd also be able to in effect, 'turn them off' with no load, whereas you can't with the Roadmaster.
If you tow alot, I'd got with the helper springs.