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I've played with both. Reel Director is a lot more powerful than iMovie. On the other hand, iMovie will generate movies in real time. Reel Director needs to render the movie. Just to give you an idea, I put together a 2 minute movie with 1 sound file and it took Reel Director about 25 minutes to render it on an iPhone 4. If they could generate movies on the fly like iMovie, it would totally rock.
 
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