Well Yahoo is best in respect of lovely style of their web site and google is best in respect of their best search result additionally as services like Adwords and Adsense etc.
I used both Yahoo and Google but I personally like to say Google is much much better than Yahoo, because its services and results are more reliable and accurate.
I don't understand why people compare them, as I think both are strong in their own areas like yahoo is nice for mailing and its sports and other news, Google is strong with its web searching.
Google search is always exact in their result. However, for professional appearance of mail box, it is better to pick Yahoo. You can easily try this out.
Both Google and Yahoo are two of the most powerful forces in the world of search engine marketing. Google is strong in its own aspects like Gmail, service tools, SERPs, news and others while Yahoo is top rated in terms of its own features like Yahoo chat, messenger etc.