The books for sure, always.
I like the films, but the books have all sorts of small humours details that just help really bring the world to life.
I also think the books have almost always got a sort of iron clad logic to them when it comes to the rules of magic and the way muggles and wizards coexist - the films are a lot looser on this logic, which i think makes it way easier for plot holes to form! x