Some time ago I wrote an article comparing major search engines - Google VS Yahoo first thing. You might take interest in reading just what I found...
Google is better in terms of usage, but my yahoo and msn traffic converts way better (PPC). Still trying to get my organic results to improve on yahoo.
Google by far. This is just my opinion of course! But when I once had a problem connecting to google (on my side, not theirs I think) I had to use yahoo and found the results less accurate to what I wanted to find and it looked a lot more cluttered than google.
I learned how to get my site on page one of Google. Yahoo? No idea. I have zero clue how to get a site ranked highly on it. If anyone knows, please do tell.