the google is the best choice to get the traffic to your site but the yahoo is also the good if you have the site that is running on the us host or provide the service in the us then the yahoo is best because there the number of users that use yahoo search engine is large
Google #1 search engine and yahoo is 2nd.Traffic depend on your key phrase .if your key phrase on 1st page on google and yahoo then your web traffic are too good.