Recently switched to Yahoo's search. Couldn't be happier. I do use a mix of both if need be, but Yahoo has a nice looking sleek AJAX interface and their search results are very accurate.
According to SEO point of view Google is the best for them B'coz all the SEO wants much as possible traffic which can only Google provide
You can definitely optimize revenue on yahoo more so then google. Google is saturated with adwords and in my opinion, has a much less ROI.
I resisted using Google for so long just to try to throw my searches to the next best competitor. I used all the hacks and tweaks on Yahoo to come up with what I thought were better search results. I still like Yahoo in a few areas, but Google's results are just always better.