Normally, the question of whether a book or its film version is better can be easily answered, because very seldom are great books turned into equally great movies. In the case of the Harry Potter books and movies, the answer is not so simple.
There is no question that the Harry Potter books are outstanding, and not just for children. Make no mistake, having sold 400 million copies doesn't mean the books aren't literature in the best sense of the word; they are. They are well written and connect with people regardless of age, nationality, or any other number of things that typically divide us. In fact, Rowling has done things in the Potter books that we have frankly never seen before.
First, she allowed her characters to grow up. Most children's literature features characters who are frozen in time; the Hardy Boys and Nancy Drew remained the same age through decades of new adventures. She also presented the adults in the stories as something other than buffoons for the kids to outwit. Furthermore, she dealt with important issues like tolerance, trust, love, and death in such a subtle way that we often didn't even notice that we were learning a lesson. Ultimately, she created a fantasy world that mirrored the real one, and made it so exciting that both kids and their parents would choose reading about Harry over playing the latest video game.
Given the success of the books and the zeal of the readers, translating the magic onto the big screen seemed unlikely; however, just the opposite happened. By the end of the film series ticket sales will likely exceed $7 billion, making it the most successful film series ever. This is due in part to the fact that the films have stayed at least essentially true to the books (with certain storylines omitted because of time constraints), as well as the fact that the main actors (Daniel Radcliffe, Emma Watson, and Rupert Grint) have grown up before our eyes. The screenplays have been well written, the visual effects amazing, and some outstanding British actors have been part of the series. As was true with the books, each new film has in many ways been better than the one that came before.
However, if forced to choose between the two formats, the edge would have to go to the books. The books give a huge amount of background information and detail that can't be conveyed on film, Rowling's dialogue is far funnier than the film dialogue, and there are numerous sub-plots the films never explore. The books also allow our imaginations to run free, as opposed to having a film director's vision imposed upon us. Finally, there is something to be said for the commitment of time we make to the books. With a movie, you sit for two hours and then go home. With the books, we spend days reading, connecting with these characters and their story one-on-one. There's nothing better than that.