In case it will make no difference and nor should compile because your method does nothing asynchronous.
If you make an asynchronous call in your code the signature of example 1 is required:
public async System.Threading.Tasks.Task<ActionResult> Index()
{
await MetodoAsync();
return View();
}
public async Task MetodoAsync()
{
// faz algo assíncrono
}
In practice, all methods that do something asynchronous need to be marked as async and shall return a Task.
In the case of Controller audience methods, this allows them to be run asynchronously by the framework.