I believe that both reading a book and watching television can serve to broaden a child's knowledge.
Of course to some extent it depends what they are watching or reading, rather than the forum itself as to what they are learning. But we must also take into account the type of learning.
Traditional learning, reading, writing, maths, science etc. Is vital for a child to learn, and thus is taught from an early age at school, but what cannot be taught is social learning, but that does not make it any less vital in a child's development.
How to communicate, make friends, be empathetic and kind; all of these things can be learned through watching children's TV programmes or by reading books, and therefore I would say that both are important for the learning and stimulation of children's minds.