I have to say that most of the time I like Tolkien better, mostly for the reasons you mentioned. Tolkien's world is more solid and developed, and there is more available to read about Middle-earth than there is about Narnia.
However, the reason that I
first liked Tolkien's work was only because it reminded me so much of Lewis' work. The first time I read the Hobbit, I liked it b/c it reminded me of CON. However, now that I'm older, I like Narnia b/c it reminds me of Middle-earth.
My love for Narnia led me into a love for Middle-earth, but now the places are switched.
Of course, LOTR and Silmarillion etc.. are written more towards adults, while CON is written for children, which accounts for my liking Tolkien's work more now that I'm older.
CON is wonderful and perfect for reading aloud to children especially, so I read those books more often than LOTR. LOTR and the Sil. are too deep to read any old time. You have to plan, "I'm going to take these 5 days off and read LOTR, and do nothing else." Whereas CON can be read whenever. As you said, it is much lighter.
If I'm reading for myself, I'd rather read LOTR. If I'm reading to someone (which I do a great deal), I'd read CON. (I love to see their faces when I read about Lucy finding that lampost, or Eustace being turned into a dragon.
) It's difficult to read LOTR aloud.
If I had to make a list of pros and cons, I don't really know which writings would win. I love them both!
"My brain and this world don't fit each other, and there's an end of it!" - G.K. Chesterton