Can I just say that the audiobook read by Stephen Fry is probably better than both?

Fry's brilliant reading covers over all but the most excruciating of Rowling's dialog to mostly make it sound good.
The book annoyed me in one very significant way that the film didn't: the main plot of the book was 'who is trying to kill Harry?' and it kept on giving clues in many directions. I don't know about you, but when the central plot is a mystery I try and work out the solution before the book tells me, that's the point of mysteries! Rowling just didn't give clues that pointed towards the real person doing it and so it was impossible to work it out. I don't mind red herrings, but if you write a mystery you should at least give your readers a chance to work it out.
I was actually really off when it was revealed who did it. I'd gone through all the characters and there were only a couple who had absolutely nothing pointing towards them, which in Poirot would mean that they probably did it, but an Agatha Christie would give a few clues, but in Rowling's book they just weren't there. Rowling is a novice writer, still only on her fourth book at this stage, and it shows. She tried to do a mystery but just couldn't work out how to give clues without giving the answer so cheated and just didn't give clues. You didn't see it coming, but that was because you couldn't.
In the film there was the eye twitch to give the clue about who it was. I thought it was a bit heavy-handed, but at least they were trying to correct an error in Rowling's text.
I stand by my judgement that she an average writer with occasional high points who happened to 'reuse' (AKA blatantly steal) the right ideas at the right time.
The film, while correcting a major flaw in the structure of the book, also missed out on creating that sense of threat and mystery to the whole thing, so while it made the mystery solvable there wasn't the same sense of mystery to it so you didn't approach it in that way. So... It's a tough call. The book had good atmosphere but couldn't live up to its own ambition due to a critical flaw in Rowling's approach to writing a mystery, whereas the film had a better mystery structure but didn't establish the same sense of atmosphere.
Out of the two I'd go for the book, because it was very enjoyable until she cocked it up, but they're both very flawed works. The third film is still probably the best thing to come out of the Harry Potter series so far. The sixth book is a good start, but it's like the second Matrix film: it's clearly only the first half of a pair, so it's impossible to judge how good it is until the storylines that it sets up are resolved. I just hope she's better at ending the whole thing than she was at resolving the mystery plot in The Goblet of Fire.