Prefer
IEnumerable<T> for inputs to public methods (more so if they are of wider utility). This gives you more flexibility, as you can theoretically pass in any collection. That is, unless you specifically want to only work with arrays for performance reasons or something. For outputs, it's often fine to return a more concrete type, especially if you are creating a new collection anyway. This plays nicely with input-output variance in polymorphic scenarios (Dog Foo(Animal) can be assigned to Func<Mammal, Mammal> in a type-safe way, assuming Animal > Mammal > Dog inheritance chain).
