I nearly always try to do both mind you. Read the book AND see the film. I can't say as I try and do it in any order tho.
I think it's interesting to read a book where the film is more loosely based on the book - eg do androids dream of electric sheep. Or I, robot.
The first time I saw a film/series and thought I MUST read the book - was The Stand. Then a coiuple of years later I got the uncut version and it was even better
p.s. I'm STILL citing the tommyknockers as a case in point where the film/series was better than the book.