It's an age-old argument, isn't it? What should you do first - read the book or see the movie?
Personally, I prefer to see the movie first. I love having an image in my head of what the characters look like. I find I still really enjoy the book. If anything, I get more out of it, because there are normally extra scenes that weren't in the movie.
If I read the book first, I'm often disappointed, because the movie hasn't captured the characters the way I would have or they have skipped out scenes I would have left in.
Twilight is a weird exception for me. I saw the movie first and liked it. Then I read the book and loved it. I then went back to the movies and saw it again. The movie was so much better after reading the book.
What do you prefer to do?