They can be used the same in the examples you gave. I think when people have multiple choices in front of them they tend to use "which". For example If we had 10 books in front of us and I wanted to know the writers you like I would say "Which writers do you like?" If we were talking about books without any books in front of us, you could still use which and it would be correct. But most people would probably say "What writers do you like?" I think if the choices are visible and in front of us we tend to use "which". Otherwise we tend to use "what". I doubt there is a grammatical rule governing their usage in the examples you gave. However I'm sure someone will mention some grammatical rule that makes it more confusing than it needs to be. In the examples you gave they're pretty much interchangeable.

Technically which is more correct than what in these cases but 'what' is more often used in spoken English.