google or yahoo, sometimes one converts better (or cheaper) than the other depending what sector it is, best way is to try both, use tracking, keep testing landing pages and new ads
PPC and getting optimized results would really depend on relative experience on keywords, competition, niche, provider. There are other smaller PPC search engine/provider that can deliver better results for top keywords vs. Google, Yahoo, etc - simply because competition from several keywords aren't that stiff as compared to top PPC engines
Id have to say Google is the king of PPC. Yahoo is Google's main competition.
Google isnt cheap or easy to use unless you know exactly what your doing.
Id be very carefull using it untill you new exactly what works and whaat doesnt.
The last thing you want is to have a masive bill and nothing to show for it.