Sorry I mislead you with my initial explanation.

What I meant by one client interface, is that I do not want to support a Webforms and a Winforms interface. The customers would be able to install the client interface and part of the payment would be the number of workstation they are using.

However you have opened up a can-worms as I can see your point in that you only need to support a single interface and update one interface in webforms - instead of updating all the client interfaces individually. - This is a good idea -