Well let's look at what the figures basically tell us. There are undoubtedly more Android phones on the market than there are Apple iPhones, but what makes the figures get confused is that IOS runs on more than just the iPhone. iPad users as well as iPod Touch users can both use the same IOS Apps and with both devices benefiting from WiFi and the iPad also having optional 3g connection, your Apps can access the internet, even if in a limited environment.
And it is this spreading of the IOS onto iPod and iPad that gives it the lead in the marketplace. Whilst there might be more Android phones than iPhones, when you include iPad and iPod Touch users, the balance does swing in the favor of the IOS.
So does that mean your App will hit a greater market share if you develop it for IOS? Not always. You have to also consider who wants to use your new App and when. If it is a game to be played at home then yes, all IOS users can use it and even iPod Touch users will have their WiFi connection there and can interact with the internet if required. In this case then a development on IOS is likely to hit more people.
However, if you want to create a more niche App, when will people use it? If it is for tracking a training session, giving latest travel information and other such Apps that people are likely to want to use when they are on the go and depending on internet access, then it is not so easy to drag an iPad around with you then (especially in an App to monitor running training!) and an iPod Touch just won't have the internet connectivity away from a WiFi hotspot. For these situations it is just possible that an Android App will reach more people.
Having said all this the question of demographics then muddies the waters. Are your users more likely to have an Android or an iPhone? I've not studied the data, but it does seem that business people and those that took on board the new technology earlier tend to go for the iPhone. Whereas people paying for the monthly contract themselves and wanting a cheaper handset with a quarter of the monthly fee will always choose an Android based model.
Ultimately it pays to know your customers. But business users seem to appreciate the iPhone whereas the main growth area would appear to be the Android market. However, with the rate that Android is increasing its market share I do suspect that before too long they might have even more users than all of the IOS users put together.