yahoo is not as good as google from the stand point that most people use google. so you want to rank on google, not so much on yahoo. But for messenger it's excellent...
i like yahoo as a messenger. I like google the most. Google is very fast in giving the updated results.
To be quite honest does anybody really class Yahoo as a search engine anymore? It's basically Bings little brother.
Bing and Yahoo are now family. The two have merged forces in order to try and gain some footing on Google. Apparently as of Mon. you need to start using Bing's webmaster tools. Google is my favorite engine but don't overlook the value of twitter for the best real time results.
For me both were same, for search engine i prefer to use Google and for messenger use I prefer skype or Gtalk. So both were same for me at the moment.