I’ve been running Google adsense and Yahoo’s Publisher network ads in an A-B test for over a month now. I had a script randomly serve either a Yahoo or a Google ad in the same place. The ads looked almost exactly identical (in color and layout) too.
The end result is that Yahoo (right now, for my site) is paying 10% more than Google ads.
Each ad network got 9000 page impressions per day, for over a month. That should be enough to be statistically relevant. But then the stats start to diverge in interesting ways.
Google’s ads got an average of a 2.3% clickthrough rate per day. Yahoo only got an average of 1.3%. BUT: the average daily revenue per day from Yahoo is 10% higher than the average daily revenue from Google.
In other words: Yahoo seems to serve (for my site) higher paying ads than Google. So even though the Yahoo ads get less clickthrough, the end result is that Yahoo pays 10% more.
Of course, these numbers will probably differ for your website. The 10% difference is statistically valid for my website, but you can not extrapolate that to other websites. The content will be different, so the ad inventory that each service has will probably be different as well.
Your final conclusion should be that Yahoo and Google pay pretty much the same, although there may be small difference (5, 10, perhaps up to 20%) in what they pay for specific websites. The only way to know for your website, is to run them both, next to each other, for a month or so, and see which one pays more for your website. If you are making 100$/day or more, 10% can end up being quite a bit of money and doing these tests and optimizing may be worth it. If you make less, don’t bother, just use one of the two for now.
And of course, you should repeat the test a few times a year. These things always change.