Yahoo > Google on some keywords, where as Google > Yahoo on some keywords. Traffic per keyword ranges vastly, so you should definitely invest in both. Who knows, in a couple years, Bing might be huge, and you might end up wishing you'd done something earlier.
Well Yahoo is good for Newbies who are new in the market . As they can get into Ranking in Yahoo before they can in Google . As its tough to get in Google .