It looks like the hitch in your picture has a removeable ball assy that can be stored in the trunk so it wouldn't stick out when not needed. If you decide to install that type of hitch, be sure to reinforce any thin metal, such as the trunk floor, that you will be attaching it to. Also keep in mind, that type of hitch design, rigidy bolted to the bumper and the trunk floor, may compromise the energy absorbing function of your rear bumper. In a rear-end accident, it could transmit the collision force directly to your trunk.
The German hitches I've seen on Mercedes were welded units that attached to the trunk/frame rails and not to the bumper.
Happy Motoring, Mark