In theory, it is possible that a movie could be better than a book. But in reality, I have yet to see a movie that was better than the book. Even those times when I have seen the movie and then read the book (Gone With The Wind comes to mind), the book was still better than the movie. I think the movie cannot capture the “behind the scenes” forces that make the book so special–the motivation of characters, the nuances of the book, the complexities of the plot. The Harry Potter movies would each be four to five hours long in order to do justice to the books, for example. I am anxious to see the movie The Help to see how it compares to the book. Another book that outshine a very good movie is To Kill A Mockingbird. The movie is great, but the book is better.
Much of a book has to be left out of the movie for sake of time and the transitions, for example, are often not well done. This was true of Harry Potter 6 (The Half-Blood Prince) where the movie did a poor job of explaining who the half-blood prince was and why this information is important to the series. But this long movie would have to have been even longer to somehow fit this into the plot of the movie and would actually have been difficult to do. Perhaps it could have been blurted out, but that still doesn’t compare to how it is discovered in the book.
I think it is nearly impossible for a movie to be better than the book, no matter which a person sees or reads first. The book can by its nature explain what a movie cannot. The movie has no mechanism to explain these things that the book can easily do in the hands of a skilled author. This doesn’t apply to original scripts for movies where there is no book. The writers can deal with transitions and motivations well because they are written into the script especially for the movie. It will be a remarkable movie indeed that can outshine then book.