The Inside AdSense blog has published a short article on how AdWords listings are ranked:

How is Ad Rank determined? While bid price (the price advertisers are willing to pay for each click per thousand impressions) is obviously an important factor, we also calculate advertiser Quality Score to make sure the best performing ads will always appear on your site. Quality Score takes into account factors such as an ad’s relevancy and click-through rate (CTR) (i.e. how likely users are to click on a given ad) because a higher CTR sometimes outweighs the benefits of a higher bid price. This ensures that your site always displays the ads representing the highest revenue potential for you.

No matter how you look at it, Google wins. The best ads will still be the ones with high bids and high CTRs.