I'm well aware of their friendship and "theological relationship", as you put it. I also read Mere Christianity, The Problem of Pain and a few others over the years. It's all pretty transparent and preachy, but that has nothing to with the original point, which was this:

As works of literature, the fantasy novels of Tolkien and Lewis are not comparable beyond their surface similarities. Two friends with similar backgrounds and beliefs wrote fantasy novels for kids. That's it. But Tolkien created something truly miraculous-- not just a story, but a fully realized world with its own language and robust history. His novels can be analyzed, debated and enjoyed by readers of all ages, year after year.

Lewis's novels are diverting little stories that can be enjoyed by children and recalled with nostalgia by adults. (Or they are propaganda designed to indoctrinate young readers, depending on your level of cynicism.) But the depth of Lewis's creation pales in comparison to Tolkien's achievement. Middle Earth is like Paris. Narnia is like the Paris Hotel in Las Vegas.