Read the book or watch the film?

In short, both.  But in the right order.

There are few pastimes more pleasurable, more peaceful, more relaxing or more satisfying than getting lost in a book.  I'm sighing with happiness at the mere thought.  I love the way you can really get into someone's head, know their innermost thoughts, get to know how they tick, what makes them mad and what makes them smile.  Over a series you can really get to know a character, like you can a real person.  You get to know how they react to certain situations, the sorts of people they enjoy spending time with, the things they like to do in their spare time.  You get more than just the action of a story: you get the internal monologue of the main character's thoughts, their own point of view on the things they see, a peek into their heart; you get a rich description of the world in which they live, which evokes the desire to live there or not; you are transported to another universe, where yours does not exist and you, as but an onlooker, are safe.

I enjoy watching films based on books I've read because they help me to envisage the characters and places described.  They're also a great way to share a story, because you can watch a film as a group and easily fit a film into an evening spent with friends.  Watching a film, or indulging in other spin-offs from a novel (for example, other books, websites or merchandise) prolongs the experience, makes you feel part of that world for longer.  Watching a film is an easy way to get a quick fix of a story you enjoy.

Two things are experienced when reading a novel: the first is discovering the action as it unfolds; the second is building up a picture in your mind's eye as the surroundings are described to you.  When you watch a film, you also discover the action as it unfolds, but the difference is that the pictures have been drawn for you.

If you watch the film before reading the book, half of the experience has been taken away.