Planning to take advantage of the sale but am uncertain as to which one to get.
As far as I can tell the real difference between the two is that the convertible bag can be expanded and the classic can't. We have two convertible bags and have been happy with them.
I have a Classic. It has shoulder straps that tuck away if not used, but does not have a waist strap. I believe that the Convertible model is "expoandable". However, if you use the expandable feature you will likely exceed the carry-on size limit.
Lack of a waist strap is not a problem when weights are keep reasonable and you are not doing something technical.