Usually the books are better since, to save time, the movie versions leave out important parts of the story and characters. Sometimes that is very disappointing.
I have watched a lot of movies based on the books and I believe sometimes movies are better than books and sometimes the movies are unable to do justice to the books. One of the best movies based on books is Clockwork Orange and One Flew Over the Cuckoo Nest.
I have watched some movies that are based on books, and some of them did not do justice to the books. The books are much better than the movies, so it is best to read them to fully understand the story. Sometimes, it is the other way around.
Most times, the movies always dilute the essence of the books and doesn't give the full vibes that were obtainable from the book. 50 shades of Gray is a typical example of such scenario.