I have an Air and haven't used it much. All three - Solo I/II, Argo and Air have removable glass stems. Air and Solo I/II use the same stems. Argo's are smaller. To me, the Air isn't enough smaller than the Solo I/II to make much difference. It has the improved draw of the Solo II and Argo, with about the same vapor output as the Argo. Also doesn't have USB charging. It does have user swapable batteries like the Argo.
Don't get me wrong. All 3 are good devices. If I had to choose one, it would be the Solo II. Max portability, Argo. Air is sort of in the middle.