Hi Tim,
An update. I put the question on the PACCIN List serve and there were immediate responses, some with really good details and a lot of consensus on the use of air ride. The problem for me is that I am not sure what the source of that consensus actually is. There is no doubt that air ride is "the standard" meaning that for museums everywhere to even think about loaning objects that its use is required.
While it is important to know the "standards" and therefore be able to fulfill peoples expectations that doesn't mean that they always make total sense. In many instances I would be more concerned about how the object is packed and especially how it is secured in the vehicle than the type of ride. Why? Because there is a lot of evidence reflecting quite the opposite of what non-packers tend to think and say on the subject. Most actual measurements taken in fact indicate that objects are not subjected to the highest levels of shock in transit but instead during the handling both before and after transit. I have seen a whole lot of graphed data that pretty consistently shows shocks measured by accerometers being fairly flat during transit and all sorts of little spikes both before and after.
What I have never seen is comprehensive comparative data on this topic. Hopefully someone will point me to some that I am just unaware of -- soon.
In the mean time I would add with some confidence that both in the case of many Museums and some art service providers there is often what they say and then there is what they actually do and the two are not always the same. Just sayin....