Comment by AnimalMuppet

I think the question is, do you know at compile time what the concrete type is? In situations where you do, use static. (I'm not sure I'd call that "polymorphism". If you know the static type it's just a function on a type, and who cares that other types have functions with the same name?) But if you don't know the concrete type at compile time, then you must use dynamic dispatch.

And you can use each approach with the same type at different points in the code - even for the same function. It just depends on you local knowledge of the concrete type.