I think it depends. Both have their issues. One thing I will say, Google's interface is a LOT cleaner, a lot nicer, and their mail app is far superior. As far as which one is better for searching, though...don't know, google is obviously the industry standard, but that could change.
I've tested yahoo against google several times,using the exact same wording,and each time,google was faster,more concise,and to the point.
I have some privacy issues with google,but so be it.
They all collect TOO much info,so what do you want from me?!
Our Google which art in cyberspace,
Hallowed be thy domain.
Thy search to come,
Thy results be done,
On my computer as it is in the WWW.
Give us this search our daily results.
Forgive us our spam, as those that have
spammed up against us.
And lead us not into infected sites,
But deliver us from Trojans.
For thine search engine is the greatest,
and the power,
and the glory,
For search after search.