I'd rather read a book because it puts me in the director's chair. My own imagination gets to fill in the images and wrap the phrases in the meanings that my own mind generates.
I find that when I watch a movie, I get distracted from the plot by the details of colors, gestures, accents, clothing, and music. Maybe I'm a little A.D.D. I just think movies give too much information.
I also like the fact that in books, I am in control of the pace. I can speed read over certain parts, and often do. I can also slow down and savor parts, or re-read sentences that are particularly beautiful or thought provoking.
Movies don't allow me to control the speed. Movies take over my mind too much.
But that's also the strength of a good movie. It can blow your mind away with the rich combination of music, colors, images, expressive characters and plot, all coming at you at once. And that can be fun.
In a few rare cases, I actually prefer the movie over the book, but that is soooo rare.
(Sense and Sensibility, the film with Emma Thompson, is one example. She used the fantastic language of Jane Austen and combined it with the most glorious cinematography-- that was an instance when my imagination was not as good as the movie turned out to be.)