If I see the movie, then I can't read the book. If I read the book, then I can't watch the movie. Whichever one I do first always ruins the other one for me!
Ah, difficult situation. Unless, of course, you clearly prefer one to the other.
I have seen films "based" on books which was so far away from the book that it didn't really matter I had read it. They basically kept the character's names and changed everything else.