There was an article about a scientific research done, and that pen was chosen as the most perfect penalty. But I would probably agree about low being more unstoppable than high.
Disagree when it comes to penalties, because all the movement of a goalkeeper comes from pushing off his legs from the centre of the goal and from that crouched position the top corner is further away than the bottom post - even if, whilst upright, their hands start closer to the top corner.
Daft discussion, like, but just my two cents!