I have had air bags in the past and now have Timbrens (not for our TT but for hauling loads in the bed) and I like the Timbrens because there is no adjustment needed. They did come with a 1" spacer to move them into position so that they are always engaged if you are always carrying a load like a TC or a utility body.
The only downside is that as noted above, the axle hits the Timbrens before the overload springs and causes somewhat harsh transitions to the overload springs when carrying a load that is not quite heavy enough to sit on them all the time. I do like the improved handling though. Even with 2000 - 3000 lbs in the bed there is little body roll in corners and the steering stays crisp since the truck is not squatting and allowing weight to be transferred to the rear.
Airbags are nice because they provide the same benefit but are adjustable although they can leak and require you to set them to the desire pressure for the current load to get optimal results.
