This is a really hard one. One one side you have a book created to show us an incredible word with so much detail and on the other you have a story about people. In the end I would go for Harry Potter because it has much more real characters (I'm taking about moral complexity). All the characters in LOTR (except for Gollum/Smeagol) are just good or just evil. Meanwhile in Harry Potter only Volemort is one dimensional. So in the end Harry Potter is better.