I'd DEFINITLEY see the movie first. I'm a RENEGADE CRAZY MAN around here for suggesting that movie is good. In fact, the movie is very good- But if you've read the graphic novel, then you're going to spend a lot of time going, "Well, where's xyz and what happened to the part where..." instead of just enjoying what's a unique and totally excellent film.
That said, the graphic novel is unparalleled, and it's a much more sophisticated take on the material. For the full story, you'll really have to read it. But watch the film first.