Google get 48% of total search volume where yahoo gets 29%. now that is not to say that Google get that percentage on every KW just the overall amount of people who uses Google vs. Yahoo.
Google is Bigger and more just search engine traffic based... yahoo people tend to hang on and check their mail / chat more than they would on google.. so yahoo tends to get more real people just browsing around in general and searching just because they have the yahoo toolbar installed from some crazy program they installed recently.. but I still believe that google is the most widely used search engine by people... I personally use google as my homepage just because there is no graphics.. very fast loading home page.. but i agree that webmasters optimize their sites for google more than any other search engine.