Yahoo may prevail over google due to higher conversion rates. I expect with the new yahoo bidding system yahoo will syphon advertisers from google.
We all know adwords sucks ass (which is why people are looking to 3rd tier for PPC arbitrage) so it is a matter of time b4 more advertisers realize this.
It's all about the eyeballs. Google has them and yahoo doesn't. Until yahoo figures out a way to get more of the search market share, they'll be a distant second.