You can use both (not in the same app!) but be carefull in what you pick!
I do my homeworks, trying both (on different apps of course) and asking around to friends.
There's not a final verdict of course. Sdk are pretty both easy, but of course iAD is completely integrated in itunesconnect, so you don't have to deal with different sites, registration and so on... but ok, is something you do once and forget, so not a big deal.
For my experience iAd give bit more money for banner shown, but AdMob have an incredible superior fill rate! I you're an "international" developer iAd is (till now, end of 2010) a poor option: my app have about 25% US customers and a forthy-somthing fill rate there. Add some more sparse filled impressions here and there, I net a 10% fill rate world-wide!
The other app, with AdMob, Italian market only, have a 66% fill rate... in a country where iAd is still not available!
So 'till Apple don't go worldwide for ads i suggest to put your focus on WHERE you plan to distribuite your apps...
Just my 2 cents
