Books or Movies?

I've been thinking about this for awhile, and I'm never sure which one to pick first. If I read a book then see the movie based on it, I spend the whole movie saying how terrible the adaptation was (case in point: A Walk to Remember). But if I see the movie first, I'm not compelled to finish the book. The only time I actually finished the book in this case was when I watched a TV version of The Shining and didn't have time to watch the end of it.

I wonder about this because when I write my short stories and novels, sometimes I can see a film adaptation in them. But if there are too many people like me, the movie adaptations would never be successful. Just food for a writer's thought...