In terms of market share, Google blows Yahoo out of the water. So with Google, you can potentially get a lot more traffic for ranking for a certain keyword. But you can still get some decent traffic from Yahoo if you rank high for a competitive keyword.
softwaretool, If you are somebody searching for information or anything, G* is slightly better at finding results you want most times, apart from that just about the same. However, for people with websites (webmasters and site owners) its all different, they prefer G* because it is used more, G* is more known now than Y!, advertised better and became the largest used search engine. Y! is not really a search engine although they have a good one, they make most of their money from other areas of their sites relying less on their SE to bring in traffic to those areas. G* on the other hand makes most of its money from their search engine directing people to their other services and advertisers.
Google is king and will remain king. Yahoo can hardly maintain its market share which is why it agreed to partner with Bing, but they both need a who redefining of there search engine models. Simple and clean and they can increase their market share along with the proper advertising.