In my personal opinion, because the book is the original story, I would tend to believe that the book cover would be a better representation only because it is the original idea. On the latter, some designers are able to really grasp a better understanding and create a beautiful design in a different way. Real people rather than cartoons or illustrations tend to grasp and engage an audience more than if it was not a person, although some movie posters still do not show real people.
I have noticed that the movie posters always tend to be of real people (obviously), but sometimes not, which could really change your perspective of the story as a whole, but you will never forget the original book design.
Looking through the differences, it is sometimes obvious when a movie poster is much more up-to-date than the book, making the movie poster more intriguing and fresh. After going through a bunch, I am 50/50.
Here are two comparisons that I found interesting:
This cover vs. poster design is a significant update; it is obvious that the movie poster was created much later than the book cover, making it much more intriguing in this case.
In this case, both the book design and poster and very similar, adding color and more of a dark, "dangerous" feel but still seems very simple and clean.
In this case, the movie poster using the face of a person is much more intriguing and interesting, bringing more depth and interest in the story by the choice of design.
