Which First? The Book or the Movie?

With the release of the movie, Water for Elephants, I was faced with the new-age chicken-and-egg question: Do I read the book or watch the movie first?

Over the years, I've seen many movies made from books I've read. The first one I think I can remember is Shirley Temple's version of The Little Princess. I'd read the book, and I remember picking out the differences, especially the ending. There have been many more books I'd read made into movies since then – Patriot Games, The Bourne...