Hello Ayaha, Google Adwords is the best ppc program in my openion.. it's easy to learn and understand. Cheers, Kranthi.
I would say Google generates more traffic but if you cannot manage it closely it can be a endless money pit whereas Yahoo! has cheaper clicks. I personally prefer Google but why not try a couple of months of each to see whats best for you.
Google is where you will get the most traffic, but you should test your offer on both. By testing, you can see which one converts the best. I have had some identical campaigns that had different conversions on each.
It seems to me that people like to clike on the main part of the page rather than on the ads to the right.