Both Android and iOS have a large developer community. Android has power of opensource and enormous free learning resources and android programming books available for app developers.
Though iOS is not an opensource software it still has strong community support and professional help in terms of learning resource and iOS Programming books readily available for developers.
Some developers may be required to do development on both Android and iOS due to a large customer base or other compelling reasons. You must also check our recommended tools for mobile development productivity - Mobile Development Productivity Tools.
Even before we start comparing, some of the things you must know about iOS and Android are listed here - iOS app development and Why developers prefer Android .
Let us see in this article which platform could emerge as a winner, given that both pose themselves as strong contenders for acceptance in the developer community.