The AirBooster measures the same height as the TurboBooster (I compared), but the depth on the base is much, much deeper. I wouldn't recommend it for anyone younger than, say, a long-legged 5 year old. My review is here:
http://www.car-seat.org/showthread.php?t=18111 . My 7.5 yr old fits well in it, but he won't last until he doesn't need a booster anymore with the back; his torso is simply too tall. The Parkway is taller, but the seat depth is very shallow. The new Cosco booster at Target is much taller than the AirBooster, but it requires a headrest behind it (um, is Cosco too cheap to build in some strength in the booster?).
We have had a problem with the AB back sliding down on its own and have sent back the original seat we bought. The 2nd seat is starting to do it as well
, so I may have to contact Graco and see if the engineers had a chance to look at my 1st seat and see what their thoughts are on it.