The difference between Narnia and Middle Earth points to the underlying difference between the imagination of Lewis the Protestant and Tolkien the Catholic. For the Protestant, truth is essentially dialectical. It consists of abstract propositions to be stated, argued, and affirmed or denied.
For the Catholic, Truth, while it may be argued dialectically, is essentially something not to be argued but experienced. The Truth is always linked with the mystery of the incarnation, and is therefore something to be encountered.
Many Protestants will argue, for instance, that God's primary revelation is Sacred Scripture, while Catholics maintain that God's primary revelation is Jesus Christ.
It was said to me once, that the difference is that one learns Protestantism but one drinks Catholicism.