In brief: Normally native developers prefers native api (Geocoder in android) for Geocoding and reverse geocoding, but to target the cross-platform implementation i think google geocoding is the better option.
In my previous post written on Integration of google map v2 in xamaron android, Avoiding deprecated google map api,Best Practice and issues with ListView in Android Xamarin, How to use Google Place API with Autocomplete in Xamarin Android, Integrating Google Account in Xamarin.Android,Avoiding ImageBitmap OutOfMemoryException and Rounded corner Image in android Xamarin.
In my previous post written on Integration of google map v2 in xamaron android, Avoiding deprecated google map api,Best Practice and issues with ListView in Android Xamarin, How to use Google Place API with Autocomplete in Xamarin Android, Integrating Google Account in Xamarin.Android,Avoiding ImageBitmap OutOfMemoryException and Rounded corner Image in android Xamarin.