Smilinize:
You asked if a writer should begin with the message or the audience, and the answer is BOTH.
Once you know what it is you want to say, you have to decide who you want to say it to, because that will determine how you say it.
Example: If you're a 25 year old CPA with a new accounting method, addressing older, established CPAs, you would do beter to take a questioning rather than an authoratitive tone. Don't say, "The way you've been doing this for the past 30 years is wrong. Here's the way it should be done." (This will definitely annoy your audience.) Instead, say something like, "Your vast experience and expertise in accounting allows you to innovate. I'd like to introduce a cost-savings method that will allow you to better serve your clients and cut down on your work load." This approach doesn't insult the intended audience, but tells them what's in it for them if they listen to your ideas.
The same applies to fiction. You have to know who your audience is in order to decide the voice you'll use to tell your story.

For more on this, see pages 1-116 of Write Publish & market Your Book (www.acapela.com/books.htm)
Patrika