Does your business needs an iOS app or an Android app?
As of 2015, "both" is usually the correct answer. The mobile market is a duopoly, and while the market share, and typical spending power of Android and iOS may vary, depending which survey you read that week, it doesn't make sense to ignore either market in the long run.
However that's not to say you should attempt to build both an Android and iOS app simultaneously. In fact, at ReignDesign, we usually advise clients to avoid doing that! Building for a single platform first lets you validate your hypotheses about your app. Is it easy to use? Is this screen really necessary? Does it meet my users' needs?
Since app development is always an iterative process, you can save time and money by first building for one platform, validating your ideas, and then later building out the second platform's app. So the question then becomes - should we build an Android app first or an iOS app first? To answer this, consider which platform allows you to validate your hypotheses about the app more easily.
First, the demographics of your target market. While I won't quote specific numbers in this blog post, to avoid it quickly becoming out of date, it's easy to find data which can help you understand the usage of Android vs iOS in your target market. Consider both market share, the versions of the OS people are running, and their average spend. For example, if most of your target users are Android users in China using Xiaomi devices, build your first version in Android (and buy some Xiaomi test devices). If your market is US businessmen with the latest iPhones, start with iOS.
Second, the habits of you and your own team. If your personal mobile phone is an iPhone, you will find it easier to build an iOS app first - even if you're hiring someone else to do all the development. Having your app installed on your personal device will allow you to use it frequently, and in a realistic way, and you'll be more aware of how "normal" apps on the platform should work. For example, if your app starts up much slower than similar apps on your personal device, you'll notice.
Thirdly, technology - some apps are easier to build on one platform. Start with the easier platform. For example, Android makes integration with other apps simpler through Intents. iOS has a simple way to integrate payments, and powerful image processing libraries.
Fourthly, distribution. Consider how you will get your app into the hands of the critical first few thousand users, and get their feedback. Android apps can easily be distributed without using Google Play, simply by distributing an APK file. The process for iOS is slower (since you need approval, even for beta apps), but less-technical users will find it easier to install an app from the iTunes Store.
Finally, talk to your app development partner. This is your first app, but it might be their 100th. In a rapidly changing mobile market, it pays to get the latest and best advice on selecting the right platform for your app.