Test them both out and see which converts better for you. In my industry, at least (tech), AdWords can deliver more traffic but there is also more competition with certain keywords. I believe Yahoo! also has a higher minimum cost per click. AdWords also has the whole AdSense side which is more robust than the Yahoo! publisher network. But it's all about return on investment ... so you never know until you try.
I have been using both Google adword and yahoo search marketing for years. The thing which exactly matters is the ROI.
The search engine performance depends on what type of product/ service you are marketing.
I observed that google adwords is giving more ROI(atleast in technology business) then yahoo. In yahoo the CPC are comparatively low than Google adwords.
One more thinng I wann add. most of the advertiser don't want to go much for yahoo search marketing because yahoo's reach in pay per click marketing is less and the advertising platform is soo complicated.
Adword becoming just more competitive so you may find rate differences from others, however as far as content matching is concerned Google shows more natural results than Yahoo! Search Marketing, this is just my personal view which may differ from others.
We use both extensively. Some phrases are just too expensive for us in Google, so sometimes we get more traffic from Yahoo for certain campaigns. But typically we get about 3x the traffic from Google as we do Yahoo.
My experience is that both are very good search engines, google obviously having the largest reach available, which yahoo is less. Yahoo however I have seen many companies have much higher conversion rates there as opposed to google, especially in ecommerce.
When you take a look at the status and marketing procedures used fo the top Google ranking websites especially in competitive niches like, Hosting, Domain Registration and SEO, you would find nearly all of the sites also using Google AdWords as it still remains among the powerful traffic pushing plans.
IMO, they provide excellent intelligence about the type, number and profitability of searches which can then be used for SEO purposes.
Google AdWords is one of the most readily measurable types of advertising available. The amount of data about your campaign available through Google AdWords reporting is incredible.
This enables the precise tracking of the effectiveness of each search phrase in your campaign. It is easy to determine what the most frequently searched phrases are, what the highest converting (conversion from visitor to lead for example) search phrases are, and what the most profitable search phrases are.