I have had both air bags and Timbrens. Had the airbags on my Duramax, I will admit, they were nice because of adjustability, but unless I kept the compressor handy, I would be out there with a bicycle pump adding air. Plus, they do require air adjustments at times, morning vs afternoon, etc...
Put Timbrens on my F350...love them. No moving parts, nothing to adjust, nothing to maintain. I ended up installing the spacer provided. I have a TransferFlow tank in the bed, even with it empty and the FW hitch in the bed, I had less than a spacer width of space...since I always have fuel in the bed, I thought it was prudent that the spacer be used. I can tell when the tank is near empty and not towing, the ride is a little rough...but this is not a luxury car, ride quality has to be diminished to pull a 20k TH...
When looking at the two, Timbrens have no moving parts, nothing to adjust, nothing to fail....while air bags require adjustments loaded and unloaded, possible air line failure over time, and you must ensure that you keep the minimum air pressures in the bags, which means they require periodic checks when not towing....I like the no fuss route...
Cale