The comparison has always struck me as inherently confused. Books are better at some things and worse at others. To say the book is better, and particularly to say books in general are better, is simply to state a preference for some aspects of storytelling (depth, digressions, prose) over others (beauty, visceral emotions, density).
I agree that a general statement "Books are better than movies" is meaningless. I also agree that generally speaking, the things you gain or lose in a film adaptation (visuals, character development, brevity, etc) can lean a person one way or another in what medium they prefer.
But when I say, "The book was better than the movie," what I'm saying is "I think the book did a better job of telling this story than the film."
I wrote a lot about this in my review of
The Martian, where I felt that the film's need to push the story into more "cinematic" territory frequently undercut the "based on real science" hook. While I thought that the book itself could also at times be too cutesy, it was at least consistent from cover to cover in its tone and its employment of scientific explanations.
So the book
The Martian is fine. And the movie
The Martian is fine. But which do I think is a better version of the story of a man named Mark Watney stranded on Mars and the rescue mission to save him? The book by a moderate margin.
I think that it can be a worthwhile thing to discuss, because if I love a story (in a movie or in a book), I am interested in knowing what others think about another version of it. (Something that is also true of film remakes of other films). Thanks to such comparisons, I feel no need to ever read Puzo's
The Godfather. But on the flip side, I hadn't heard of
Little Children before watching the 2006 film and I really ended up liking the book, which is more overtly funny than the film. (I would actually say this is a rare example of me NOT having a preference, because I think that the film and book versions are different but equally strong versions of the story).