This is the endless argument in mobile development. A couple of years ago iOS owned this debate, but with the revamp of Google Play, the improvement in Android smartphones, and almost all major apps available on both platforms the decision has become more difficult.
There have been countless arguments on this debate, but it really comes down to one thing. What platform does the majority of your target audience use? If you make a killer iOS app but your target audience is primarily on Android you'll have made the wrong decision. There is no "correct" answer here, so take a look at these pros & cons for iOS and Android: