If all you have is a single page with a long sales letter, then definitely use adwords. When people are in a buying spirit and they see your ad, they can come right to you.
However, there is a since of credibility, trustworthiness and authority that can be perceived when a site ranks high (or at all) in the organic listing methods.
If the same site came up twice in the search engine on the same page, once in the #1 spot for organic and also in the #1 spot in the adwords section, I believe there would be an edge to getting the conversion from the organic method simply because of the impression the searcher had about how they found you.
Think about your own experience. When you look over to the right side of the Google search results page, your mind is probably already beginning to imagine the huge, colorful, "Buy Me Now", sales letter tactics to get money. And then, sometimes you reluctantly click the link and clinch.
"What's it going to be? ...What's it going to be?" you ask yourself as you cross your fingers.
Imagine if more people simply marketed their information site that wasn't selling anything. As this caught on, more and more people would click those links I believe.
In summary, the impression that you are sending your searchers is a BIG part of difference between the two.....IMHO.....