This topic has been reviewed quite a few times. It depends on what you want in the end. With iAd, the eCPM is high, but the fill rate is low, especially from the USA. On the other hand, with Admob, the eCPM is low (er), but the fill rate is higher than iAd.
What you can do is get the best of both worlds and use something like AdWhirl to show both types of ads, rotating through different ad networks
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