What I would recommend depends on what kind of App you want to write. If you're looking to make a game I would highly recommend MonoGame. You can just develop using XNA and Visual Studio. Once you have the app/game near completion, its very easy to port the code to MonoTouch(Xamarin) with MonoGame and build for the App store. Being able to easily port to Android, Windows Phone and XBOX is a nice bonus as well. Mike.