I would agree that both solutions feel a bit hackish, but currently these are the only ways to hide modal RadWindows. Both approaches work for in-browser and out-of-browser applications. Setting the ModalBackground to null can be done using just a binding and, therefore, I believe it's easier to be understood by other developers. That's why I would chose it, but in fact this is a matter of personal choice.
All the best,
the Telerik team