Two quick comments.

On the iPhone very few advertisements are iAds. They are mostly from other vendors so in that regard I can't see why there should be any advantage in advertising on other platforms. Surely a Google ad. will cost the same wherever your place it?

Secondly. I hate ads. I would much sooner pay 80p for my app and not have a bloody big banner that I can accidently hit that will then take me out of the app that I am in. I hope that those "free" apps on Android all have a paid for free alternative.