I think the AS version is a supplementary system, working with rather than in place of existing suspension. I think it is probably most suitable where there are problems with loads affecting the ride and possible tail end lowering as a result. in this case generally the ride will be harder/firmer as you are adding additional "springing."
With full air suspension the existing springs are removed so you rely totally on the air.
My problem was my ride was already too firm, so adding to this would have been the opposite of what I was looking for.
I think - but stand to be corrected - that only VB
offer full As on a TAG.