OK my two cents worth....I think both searches are good but favor different results. My sites have traditionally done better in yahoo but I now am doing well in Google also. I find that Google tends to favor the "big boys" who cover lots of territory (larger information categories) but don't have as much specific content to that which was searched for. While Yahoo is quicker to give the site with more specific content the rankings.
Google is always going to be my top pick for searching, because it's obvious they return the top results. However, I prefer Yahoo for news, interesting articles, etc. as well as Yahoo Answers, Yahoo Groups, and Yahoo email. I love Google for so many other things though, mainly all the free services they provide - Google Webmasters, Google Earth, etc. Overall, Google leads.
It is not easy to rank well in Yahoo and terms of search results I think they give organic results for the site.