I've tried (and still own) both. I originally liked the AirHawk, but eventually preferred the sheepskin covered medical gel foam seat. Both are easy add-ons for a Vision. I'll say up front that I really use the stock seat most often. I travel with the gel seat for a change of pace and feel after fuel stops. The trick to the AirHawk seat is using minimal air in it. Most folks initially use too much. It ends up needing only a bare minimum of air - just enough to get your butt slightly above sinking through it. That is how it works best, but I still prefer the gel seat with sheepskin cover overall.
