Motion control vs face swap
Motion control regenerates your entire body into the movement of a reference clip, while a face swap keeps the reference footage exactly as it is and replaces only the face in it — so motion control needs a full-body photo and a face swap needs only a head shot.
What each one keeps
A face swap keeps the original video. The body, the clothes, the lighting and the background all belong to the reference clip; your contribution is a face composited into it, frame by frame.
Motion control keeps nothing but the movement. Body, clothes, background — all of it is generated to match your photo.
Which photo each one needs
Face swap: a close-up, front-on, well-lit head shot. Nothing below the shoulders is used, so nothing below the shoulders matters.
Motion control: the whole body, head to toe. A head shot cannot drive a full-body routine.
How each one fails
A face swap fails at the seam. When the head turns far or the lighting on the original face was very different from yours, the join at the jaw and hairline stops being invisible.
Motion control fails at the extremities and over time. Hands and feet go first; identity drift comes later in a long clip.
Which to choose
If the point is the body — a routine, a walk, a spin — it has to be motion control. If the point is putting you into a scene you could not be in, and the body does not need to be yours, a face swap is faster, cheaper and usually more convincing.