Finally realized Im responding to really old posts, but the librarian in me can't resist answering! Both were brilliant! Thousand Splendid Suns is the pick by my friend. I think it's mostly because of being able to relate more to the female characters. I love how that book is set up. The first two sections of two different characters and then those characters interacting in the end.
However, I might have to choose Kite Runner as my favorite, probably only because I read it first and it really haunted me. I dont normally read these type of books (the latest bestsellers or multicultural) but I just found myself fascinated about the commonalities between human beings. I felt that I left that book with a better understanding of the Afghan people. Not to mention just the mistakes we all make as humans that haunt us for the rest of our lives.