It's obvious that Yahoo! Publisher Network offers higher pay and has a good database of diverse advertisers, but all-in-all Google's Adsense has the larger database of publishers because of the amount of countries Google can target. Yahoo! Publisher Network is definately something that I'd be interested in getting involved with as a publisher.
Indeed, lo and behold we have Yahoo ad up there in the top right. So I tried to sign up for Yahoo but: a) it took me about 20 min to find the damn sign up page, hidden among, well, the fat mess of other crap Yahoo does. A yahoo search does not help btw. b) Yahoo is calling their publishing network a "beta" program still c) It requires that I provide a "valid U.S. Social Security or Tax ID number, and web site content that is predominantly in English and targeted at a U.S. user base." Which is bullshit. Clearly Google has them whuped internationally then, and I'm glad.
I have been running ypn on four of my sites now for about 6 months that were showing adsense ads before that and all four of my sites have increased revenue of about 10% using ypn.
AdSense was super for me, they had very high ctr and great relevant ads, then 'SmartPricing' hit my sites......... 1, 2, 3 cent EPC..... I love YPN, but now that is earning me little also. I am going to try ad targeting today
ypn pays much higher then adsense on my end as well. google as better targeting but i still make more with ypn and yahoo send the checks on time google takes forever to send you your money.