iOS App Advertisement choice : iAd or AdMob ? what will you choose , iAd or AdMob ? and what is the reason behind ?

I'm now using iAd, as I think Apple may block AdMob anytime, as Apple wants to protect its iAd. Is it true?
It is easy to combine both in the same iOS app.
This URL will provide you more information about this issue.

Summary: 
You try to load iAd, if that fails to load or something else happens - you can fall back on AdMob.