That's a tricky question, OP. I'm guessing Yahoo because a lot of Asian countries tend to use sites like Yahoo/Ask. Especially with China recently banning Google (not sure if they uplifted that ban), you could imagine how many people are using Yahoo from China. Yugo,
if you want to promote the site in the us the yahoo is the best choice to get the traffic for your site in the us there is great number of the users of yahoo in the us
ya i guess also that Google is the best traffic generator in US, but when i see the signup email id's of users on my site they all are yahoo or hotmail users, some are from aol. From 200 user about 170 have yahoo email id's and rest others, no one is from gmail. This point is confusing me whether to choose Google or Yahoo.... but again i rely your replies and move towards google.
We all know Google send more traffic , difference between Yahoo and Google traffic is huge one, Google traffic is about 500 times more then Yahoo , mean if Yahoo gives 14K visitors for any keyword Google gives 70K visitors for same keyword. Also Yahoo now showing bing searches so why optimize for Yahoo, think about bing now.
From my personal stats Google comes first. Funny enough is that I had asked this question on a forum a long time ago and nobody gave me an honest answer.
both can give you a good traffic.. though google can give you far better compared to yahoo.. so I suggest you go with google or both..