Ah the long debated question of this generation. xD
For me, I'd generally lean to the movie adaption of the book. Reason being, film is something that has always interested me. Despite Hollywood messing up a few good moments in the books, I think for the most part they have done a pretty good job. Humans have been turning works of literature into physical/visual adaptations for thousands of years, and I think it's the same idea all around: A moment is more defined when you see it happen v.s. when you imagine it happening.
Books are very subjective, and allows the reader to envision the writer's words in their own way. Yes, I won't argue that sometimes imagining the scenario in your head with character looks, setting, conditions, etc. could be a way cooler depiction then what the movie directors choose to go with, but I think putting words to a picture helps people to understand and enjoy more.
There's also another side to this too. Since no one reads the same book in the same way, you could have a totally different understanding of the book than the director who's in charge of adapting it. You're seeing the movie through his/her eyes instead of your own, and generally that could lead to a distaste for the film/story itself.
But let's also not forget that a writer can write as much as he/she wants. A director has a very hard task of fitting an entire book (Most novels chronicle a few days, even weeks or months) into perhaps a maximum of 2 hours. Not only that, but the director also has to worry about actors and staff, keeping the movie within budget, sounds, lighting, CGI, and a whole plethora of other things a writer doesn't necessarily have to worry so much about. Sure, a writer can spend loads of energy planning out characters, plot, etc., but they have no creative limits, whereas a director does.
On the contrary, there are plenty of movie adaptations of books that have been horrible. But I don't think that it's ever the movie's fault or the director's fault. There are so many culminating factors such as the inability to cover a large plot in a small allotted time, actors who don't fit the character, a few bad/cheesy lines in the screen-write, etc. Which is why it honestly really depends. Then of course, you have to take in the personal taste of the viewer and all things of the sorts. It's a very hard choice, but I think the movie's where it's at for me.