iOS apps are where it all started, while it may not have as many users as Android, successful IOS apps can be hugely rewarding if executed correctly. IOS users tend to be be the biggest app spenders, so a carefully crafted mobile app, on the iOS app store can lead to great steady stream of additional income.
Being the largest mobile market, Android has grown at an incredible rate, if are looking to target a large number of users, building a mobile app on Android is the choice for you. Your new shiny app, once completed can be on Google play store on the same day it is submitted.
While building a BB10 mobile app may not be practical for start ups, Android apps can be ported across to BB10 based devices in a few click, so why not add your shiny new app to another app eco-system, for a low cost, as Blackberry app store is far less fierce, having your app on their store could boost your brands aware via the niche mobile platform.