The answer to this is a question would be for everyone different, but I'm going to give it a try.

When you're reading a book, you have to imagine what the character(s) and/or place(s) would look like. By doing this, you'll create your own interpretation of what the writer has written. On the other side, when you watch a movie, you'll jump in a world that's already interpreted by someone else.  So you will need less of you own imagination, and your own contribution is minimized.

Few examples:
- Jurassic Park:
I've had the chance to read the novel written by Michael Crichton, but I had already seen the movie directed by Steven Spielberg. So I've had an idea about what the characters would look like (it's easier to fantasize/visualize when you've already seen the movie . But despite the amazing special effects (the dinosaurs looked almost as if they were alive), the book was better!

- Harry Potter:
Like most of the people I discovered the young wizard Harry, by watching the first 3 movies. But because they were so spectacular, I couldn't wait until the next one was released. So I started reading the books and my opinion is the books were better than the movies.

- Lord of the Rings:
This is the same story like Harry Potter, I discovered it when I saw the first movie in the theatres, but I couldn't wait until the next ones were released. So I started reading the books. There is one thing that I can remember which was very disappointing about the third movie: The part where the 3 heroes (Aragorn, Legolas & Gimli) along with a ghost army are saving the (under siege by the orcs and nearby capitalizing) city of Minas Thirith, was a real page turner in the book. However in the movie this specific scene was disappointing; our heroes arrive with the ghost army and whoosh, bye bye enemies.

Conclusion:
In both cases, you're not sure if the interpretation is correct with the image that the writer has created. But sometimes (for example Harry Potter), the director has to work closely together with the writer to create the image, that the writer had in mind when he or she was writing the story.

All about the money:
Sometimes you notice that it's all about the money when they are making films. A good example is the Hobbit trilogy. I've read the book before the movie and when I heard about the idea of making a movie about it, I was very excited about how they going to do it. But it was not one movie that they wanted to make, but a trilogy. And from that moment I was wondering how they are going to make 3 movies of approximately 1.5 to 2 hours from a book of 300 pages.

Film adaption of a comic:
We all know batman, but it isn’t until 2005, when the movie "Batman Begins"(with Christian Bale) was released that we finally meet the dark mood that's surrounding Batman. And how he has to struggle trying to save everyone even when it's not possible. The mask and cape are not a blessing but a burden that he must carry alone for the legacy of his murdered parents. And that's how the real fans know him from the comic books.
This is the main reason why comic fans are reserved about making a film of their beloved comic book character. But I think that Hollywood now understands to keep the characters like they meant to be.

My conclusion:
I like to read my books like the old fashion way on paper and my movies in HD quality.