I personally think Android might have a bit of an edge here.  Looking at figures, I believe there's more Android sales than iOS: http://blogote.com/2010/android/android-...hones.html

The biggest problem about the iPhone is that Apple are targeting the premium market, and there are many people who simply aren't willing to pay so much for a phone.  This is a place where the potential for a much cheaper devices comes in, eg Android, Symbian etc.  Apple do have the iTouch in the lower end, but no real cheap phone.  If they did manage to penetrate the lower end market, I'd probably say iOS without much of a doubt.