When it comes down to it, although I was introduced to Mr. Tolkien far earlier in life than to Mr. Lewis, I must vote for Narnia. "Lord of the Rings" did provide role models of heroism; but signs of Mr. Tolkien's Christian faith were buried so deep down that they might as well not have been there at all, for all the good they did me at first. (Note that I knew nothing back then about the Silmarillion with its revelations about God as Eru Iluvatar.) In fact, some aspects of the story were positive hindrances to my spiritual enlightenment as a teenager--for they led me to conceive the half-articulated question, "What kind of God would create a world and let Sauron do whatever he liked in it, without doing _anything_ to stop him, and then let the very salvation of that world depend, NOT on God Himself bearing the burden, but on the roll of the dice that a hobbit who was _not_ God incarnate _might_ make it to Mount Doom and throw the Ring into it?"
The Chronicles, on the other hand, put evil in its proper place vis-a-vis good, i.e. there IS a force of good clearly mightier than the White Witch or the Calormene empire.