I dont know about earnings, but campaigns we have running on adcentre return a much lower cost per conversion than adwords. Prob due to the lack of traffic! So nothing will beat the mighty G in terms of delivering results, but adcentre generates a nice steady flow of very low cost conversions.
google adsense is better because it has more advertisers, yahoo only for usa and canada, and bing not much advertisers.
And also, the publishers niche is should be in their favour. I give questions regarding microsoft and other certification exams for free. So they tell that, i am not eligible or website has illegal content. So i am going to stick with adsense.
Dittos on what SJR said...lower cost but not quite as much traffic although I've noticed a significant increase with Bing getting more popular. last month I got more clicks than I have in the nearly 18 months prior using them and I thought the costs reasonable. I love Google Adwords though and plan to go back if finances improve - darned recession kicked me down a bit. (but I never stay down...lol)