preparing a speech is a sure way to hit a wall, because you are so focused on what you want to say that if they throw in a curve ball, you'll be like a deer in headlights.
Know the company, know yourself, and be confident.. Confidence and assertive attitude gives off the vibe that "yes, I can take what you throw at me" and that's what people want to hire most of the time.
Also, reciting a memorized speech is not confident or assertive. It shows that you can't handle on the fly changes to situations.
That's just my 2 cents however.