Whether you should choose a camera mounted or freestanding prompter system depends on if the camera will move.  A freestanding prompter is independently supported in front of the camera and not physically attached to the camera and tripod.  So, if you pan, tilt or dolly the camera to follow your subject,(i.e. walk and talk), a freestanding prompter will not work because it will not stay in front of the lens.  When a prompter is camera mounted it is connected to the tripod head and moves with the camera in any direction.  Although the frrestanding prompter is not appropriate when the camera is moving, it is very effective for a static head shot.  It enables larger prompter than otherwise would be possible when using lightweight camera support (low capacity tripod).