Here is an interesting question: If you have 20% space to spare on your web page, what would you choose to get more money?
The choice might not be easy. Fortunately, there are methods to find out:
We are in an interesting position at ProductWiki as we generate our revenue from two different sources of pay-per-click (PPC) advertising: Google AdSense and Merchant Listings. These ads show up on all of our product pages (never at the same time) and each type of ad gets approximately a 50% share of our page views across a broad spectrum of products.

I've compiled data contrasting the performance of and Google AdSense on ProductWiki taken from a one week period at the end of last month. AdSense
Clickthrough rate (%) 29% 6.5%
Revenue per click ($/click) $0.21 $0.19
eCPM ($/1000 impressions) $59 $13

Taking a look at the most significant of these figures (eCPM), outperforms AdSense by a factor of 4.6!