The two websites are very different. If you're talking about search results, then Google is the guaranteed winner. They update their search engine algorithm very frequently. They care about their search results, and what they produce. Their email in my opinion is also significantly better because there are tabs separating your Youtube results, Promotions, and your regular email.
Yahoo however, is very different. I dislike how Yahoo looks; their site is too cluttered. There are unnecessary tabs such as Horoscopes, Scores, Quotes, etc. which is useless in my opinion. The email is a decent system other than the fact that you receive a lot of spam if you're using Yahoo. Overall, Google is the winner here in terms of email, and Yahoo is decent.