Google all the way, definitely! I used to use Yahoo a lot in the past (before Google came around), but it has increasingly lost relevance when compared to Google's much better search capabilities. The switch was gradual as I discovered that I got better results with Google than with Yahoo.
By the way, did you know that you can get more options with Google search when you click on the "Show options" link in the top left of the search results page below the Google logo? There are quite a few cool functions there, like the Wonder Wheel and the timeline view. Try them out!
