Thanks for your feedback. That's true, I should have just put the link to the essay on the post itself (http://brainstorm-services.com/wcu-2004/fairystories-tolkien.pdf).

I think you are right that different Secondary Worlds are more popular with different people. Tolkien preferred to keep Middle Earth separate and many Tolkien fans appreciate that, while Narnia fans see nothing wrong with including aspects of the Primary World in his Secondary World. Personally, I think that Narnia is even more intriguing because it includes aspects of classical literature and myths which are familiar to us, but made new and magical in the world of Narnia.