Which is more powerful the written word or the spoken word?

Images are more powerful than words

because: Turning words into images is easier for people to remember.

because: images reach a wider audience, even those who cannot read

I would argue that the spoken word is stronger, in that the speaker has the ability to pace and adjust to the audience in real time. Prose, may be misunderstood or misinterpreted by the reader.

Poetry and plays (both meant to be heard more than read) are for many considered the highest forms of literature.

The spoken word also has the ability to reach more, simply due to the fact that some are illiterate and you can still reach those that can't read (or just can't read the language, such as second language learners).

It is also noted that a great deal of our understanding of anything comes from body language and tone of voice, both lost to the printed page.

Omniglot has an interesting comparison between spoken and written language here: http://www.omniglot.com/writing/writingvspeech.htm

Spoken word.

Whichever is communicated first, and most thoroughly.