Google and yahoo are the best among all search engines incorporated worldwide until now. Both compete with each other for providing better searching experience to their customers, web publishers and advertisers. But Google is far ahead of Yahoo in terms of revenue generation, customer base and product innovation technology. For the time being Google is the best but we can't predict future competition.
I would go with Google. Google is more sophisticated and user friendly than Yahoo. Further Google had recently seen its profits soaring while at the same time Yahoo's share values where reaching its lowest rates.