Difference between returning Void and () in a swift closure

Same. It's just a typealias so it works exactly the same.

typealias Void = ()

Sounds like Erica Sadun and Apple are trying to stick with Void: http://ericasadun.com/2015/05/11/swift-vs-void/

If you look into headers, you will see that Void is typealias for (),

/// The empty tuple type.
///
/// This is the default return type of functions for which no explicit
/// return type is specified.
typealias Void = ()

You can verify that using playground like this,

let a  = Void()
print(a) // prints  ()

Update

If you wish to see the declaration of Void in header file, in the above code, type the code below in swift playground or xcode editor like so,

  let a: Void  =  ()

Then, cmd + click on the "Void" keyword in above expression, you will be taken to the header file where you can actually see the declaration for Void.

The document has been updated with more information which is like this,

/// The return type of functions that don't explicitly specify a return type;
/// an empty tuple (i.e., `()`).
///
/// When declaring a function or method, you don't need to specify a return
/// type if no value will be returned. However, the type of a function,
/// method, or closure always includes a return type, which is `Void` if
/// otherwise unspecified.
///
/// Use `Void` or an empty tuple as the return type when declaring a
/// closure, function, or method that doesn't return a value.
///
///     // No return type declared:
///     func logMessage(_ s: String) {
///         print("Message: \(s)")
///     }
///
///     let logger: (String) -> Void = logMessage
///     logger("This is a void function")
///     // Prints "Message: This is a void function"
public typealias Void = ()