Neither one is better than the other. It all depends on what is being presented. The question is too vague. You could ask is spoken poetry more powerful than written poetry for example, but even then it is very personal. For one person, a written text can be very powerful, since they are able to infer what the words actually mean out of their own personal opinions and thoughts. It may not be what the author originally intended the words to mean, but the piece is still powerful. However, to another person, those same words have no meaning because they are not able to connect to them. But once the author reads them out loud and emphasises whatever (s)he thinks is important, and presents it in a way which makes the words more powerful to the second person. You can't say that spoken or written words are more powerful, because you cannot speak for someone else. Also, as it is different from person to person, it is different from piece to piece. MLK's "I have a Dream" speech is not as powerful if you have only read it. But once you have heard him perform it, it comes alive. However, in my opinion, authors such as James Joyce are not as powerful when read aloud, because the effect is lost.
It all depends on the words and the person. In some cases spoken words are more powerful, while in others written words are more powerful.