Book, definitely, then you can imagine it for yourself. If you watch the film first, you will know what's going to happen when you are reading - and scriptwriters do sometimes take licence with the original story from the book.
I like to read the book (after all that's the version that the Author has imagined in the first place) before I see how the film studio have (usually) mangled it to fit a certain running time, or for a particular audience.
I'd watch the film first, as I've always found if I've read the book first , as I watch the film I get annoyed at all the parts they miss out or change, I know they change it but sometimes its a huge difference.
They do this with tv series too. The Dexter books are different altogether from the tv series. So different that I have stopped watching the series and now just read the books.