What are you building? Whom are your target customers?
Based on marketshare Android is growing faster than iOS, but you also need to know what your target customer is and that should allow you to make a decision. Currently we have 59% of our users on Android.
Here are three suggestions to target both (or more than both):
Another alternative to target both is to go with MonoTouch or Xamarin studio, which allows you to build native applications that target several mobile platforms using C#. That way you have one language to handle instead of Dalvik and Objective-C and you can work with similar experiences across both platforms.
Also Telerik has recently released software to build mobile apps for several platforms, this would have similar benefits. However I haven't tried their particular toolset.