It's a hatch back hands down, because it doesn't have a truck lid, the entire back end including the rear window opens...that designates a hatch back. The fast back is the shape of the back end and still has a trunk lid.
You would have to take a coupe and somehow slope the back window down or something stupid to make a fast back, and it would look real stupid. Or you can weld up the hatch along the window and make that small little metal area underneath into a trunk lid...then you can call it a fastback.