The book usually relates the facts and the Movie dramatizes them.
Books can go into detail about environment and history, etc. while a movie must find its inciting incident early in the story and follow pretty tight structure points in its 3 acts.
The commitments to time and structure can limit the content causing the script writer an often difficult job.
If both are available simultaneously, see the movie first. It seldom spoils a movie to read the book it came from first. However, if you like the element of surprise when watching a movie, then see it first.
Remember, a book is the theatre of the mind. Your protagonist may look quite different than the one on screen. A movie fills in areas left to your imagination in the book.
I’ve long advised young film makers who want a leg up when starting out in the biz to “write the book.” The book provides the guide for the movie and the original author is more in a position of power to see the movie through and even participate in production in some cases. The bigger the hit, the more power you have.