There are more and more tools coming onto the market that will let you build for one focus and generate others. Better yet, if you target an HTML5 Web App, you should be able to run on both devices as well as Windows Phone 8 should it take off.
Right now the Android market is growing faster than the iOS market. Additionally, there are a lot more stores to post an Android application into. If you build an Android app, you can also target Kindle, Nook, and other eReader devices, which could give you even more of a market.
Android is something that can be available in Phone that costs $200 - $600 or even less whereas same is not possible with an iOS Phone. But the users of Android (in numbers) are more so if you go with Android app development, your app can reach more users (It may vary from country to country) whereas if you go with iOS, it will cost you more but if you create an app for an iOS then you may get paid even more.
Both are in boom , it will depends more on your interest too.
Mobile development is primarily carried out using native toolsets for different platforms, such as iOS (iPhone / iPad), Android, BlackBerry, Windows Phone and Symbian.
Android apps development in current are famous more then iphone apps.
Android is well liked by some developers because it provides an opendevelopment platform.
On the other hand, developers fare pretty well when writing apps forthe iPhone, at least at the outset. Since the iPhone operating system is a closed system.
As per my opinion both is good, yes, both has the unique specialties with it. In iOS the standard maintained by the apple is incredible and the drawback is its closed one, ie not a opensource one. The majority market share is in United states alone, nothing much in other countries.
Taking android on the other hand, it has the majority market share than iOS, & its Opensource as well. Many of the mobile comes with the Android OS only, the drawback is its quality and less no of apps & open source.