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Or, if we disregard costs, C# and use Xamarin's (http://xamarin.com/) products.

Same goes for C# and Xamarin, if you already know C# and enjoy coding in C# ...

If you don't know C# I see little reason to buy a Xamarin license (I know you can get started for free which is great).

Code sharing between platforms could make it a worthwhile investment of time and money. But it's quite costly if you are just starting out.