Inevitably this question comes up during initial conversations with a client or a mobile strategy session. As both iPhone and Android evolve, and the market and user base grows, this question is becoming more complex.
Here are a few other points that should influence your choice.
Design - Better in iPhone iOS?
iPhone, in my opinion offers sleeker design options and is generally expected by consumers to be the first app a business provides. Historically that has been true, however not as much today. Similar User Interface designs and sleekness can be obtained in Android apps, especially if Android 5, codename Lollipop is the targeted Android version.
Reach - The Question of Quantity or Quality
In terms of sheer numbers of worldwide users, Android wins. We hear the market share numbers all the time. But that should not be a deciding factor, because not all Android phones included in worldwide market share numbers are considered smartphones. That is phones that support at a minimum, an app store, GPS, camera and maps.
The advice I give to all clients is that iPhone drives design and usability, so it’s the better choice to be the first app a business brings to market.
At the end of the day, if your app is for consumer consumption you really need to support both platforms since realistically they split the consumer market and I don’t see that changing anytime soon.