Use Class instead of Interface that is what I discovered after all my research.
Why? A class alone is less code than a class-plus-interface. (anyway you may require a Class for data model)
Why? A class can act as an interface (use implements instead of extends).
Why? An interface-class can be a provider lookup token in Angular dependency injection.
Basically a Class can do all, what an Interface will do. So may never need to use an Interface.