If you look at the exterior of the Constance bag it is similar to the edged designs of the Kelly Sellier. I have been told by my store manager that sellier style bags are a lot harder to produce in terms of time and craftsmanship than the "retourne" styles of the birkin and kelly. Therefore, there is also a price difference between the retourne and sellier kellys. The SM also told me that making the kelly sellier and constance is reserved to the craftsmen who are veterans.
On the other hand, I own both the Constance and Kelly wallet and I no longer use the Kelly wallet on a daily basis. The 2 closing flaps drive me INSANE whenever I need access to the goods inside. Although Constance wallet is a lot easier to use, my most used wallets are my Bottega Veneta & Chanel zip around wallets. They just make a whole lot more sense to use throughout the day....