Aha:
The primary functional difference is thatflatten is a method of an ndarray object and hence can only be called for true numpy arrays. In contrast ravel() is a library-level function and hence can be called on any object that can successfully be parsed. For example ravel() will work on a list of ndarrays, while flatten (obviously) won't.
In addition, as @jonrsharpe pointed out in his comment, the flatten method always returns a copy, while ravel only does so "if needed." Still not quite sure how this determination is made.
