The question whether to define an Interface or AddIn has two aspects.

1. The interface is intentionally a simple and therefore limited construct (see OPC 10000-3). Therefore, depending on what needs to be defined an interface may not be an option.
2. An AddIn implies a grouping Object with a standardized default name, whereas an interface can be deployed without grouping Object. Of course, even with an interface an additional grouping Object can be created and the interface gets deployed on the grouping Object.

Therefore, it is recommended to use an interface if something rather small and simple is defined (with maybe a handful of Variables and Methods) that potentially should be deployed directly on an ObjectType (without grouping Object).

In all other cases, it is recommended to define an AddIn. Potentially, an Interface can be defined and in addition an AddIn using the Interface.