Oh, I think the books are always better than the movies. The producers of the movie is always having to cut lots of things out to fit the standard 2 hour time limit in the movie. The only exception I can think of is The Lord of the Rings. The producer there did an absolutely spectacular job of fitting in most of the story from the 3 books but then again was really three movies released over three years. There is not that many books that I think the public would wait such a long time to see the whole story.
The reason I prefer DVDs though, is that I just don't have the time to sit around and read a book unless of I'm laying around vacationing at the beach. But even then actually prefer reading nonfiction and learning something.