Here is an important tradeoff between speaking and writing. Writing allows the user more room to color the information with their own experiences. For example the words "a very long time" conjures up some concept for a reader -- but when a listener hears "a very long time" from an old person versus a young person they may hear those as two different things. It's important and possibly absent information here that "if [you] didn't say it, [you] didn't say it." Inference is an incredibly powerful tool of persuasion and used often.
Gesture, tone, emphasis, etc... can give an incredible amount of information that people may process quickly and easily. It may alleviate some confusion created by the readers personal interpretation of words.
I think this thread well covers benefits and drawbacks of written versus spoken word. For me, I think of it like I think of various technologies available for a project. There are different tradeoffs and I must consider what I am trying to accomplish, what the pain points are going to be, who my users are, what is available, and make a careful choice.
