Google gets more search vollume.
BUT, that doesn't mean will get more impressions for a campaign. At least not easily.
Here's why:
It all has to do with Google's quality score and love for bitch slapping hopeful affiliates.
Let's say you are selling Gay Forums, like this one. So you create an LP all about how your forum is gayer than any other forum, perhaps the gayest in the world.
Cool. So you bid on Gay forum related terms on google and yahoo. At this point Google will likely give you 2-3 times more impressions.
BUT, lets say you want to get more impressions for other types of semi-related keywords, say Homosexual Webmasters. On Yahoo, you can just go out and bid on those terms and write an ad that appeals to those people (the Homosexual Webmasters). You can do the same thing with tons of other semi-unrelated terms.
But if you try that on google you are likely to have a poor quality score, or even get slapped with $5.00 or $10.00 bids if your keywords are way off. To get impressions for all those words you need to have targeted LP's for each term.
So while Google gets more search vollume per keyword, it may be easier (and cheaper) to get more traffic from a broad range of keywords on Yahoo.
Make sense?
So if i just had to stick with one SE, and didn't have time or money to make new LP's all the time, i would go with Yahoo. Once you get that down and start making $$$ THEN try to tackle adwords (it will be a life-long task).
just my $.02