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Running on Google and Overture you will not get much overlap. A lot of advertisers do that. However, when you get down to the garbage engines, kanoodle, goclick, 7search, blowsearch, etc. They feed a lot to other "click farms" and you waste your money.
You have the two PPC search engines you need for now.
Overture's conversion is 3 times more. I prefer the Google control panel by miles, it's fast and simple compared to Overture, but we've found in our industry (travel) Google searchers grab info from your site for research purposes and then scram whereas we *feel* Overture's search engines are used by buyers more than researchers.
Install the conversion scripts for both G and OV, you'll soon have your answer.
We had turned off Kanoodle and FindWhat for a period and then turned them back on for about a month and recieved zero conversions. I honestly believe they have WORSE traffic now than this time last year.
We're pleased with both AdWords and Overture though.
Also of note, we've found MSN to convert terrifically NATURALLY. Google was a little better than Yahoo, as well, but nowhere near MSN. AOL and Froogle convert nicely, as well.
But I would not use any of the other garbage PPCs ranked below them, except maybe FindWhat, and be careful with it.