In modern times, many people who study and actually have had full contact fist fights use a vertical fist, Bruce Lee was one of them. I am another. I personally think there are tot many variables to say which provides more power, but I know from my actual street fights that you are less likely (only less likely) to hurt your wrist and hand with a vertical punch. I have seen many a knuckles broken on the human forehead. I have seen many wrists buckle under the force of a punch hitting a body.
Why did they use a vertical punch? Because punching with a vertical fist provides for two things – fewer places in the arm for energy to “get lost” (like a bent elbow or wrist), and more protection for the arm as a whole and the fist. More kinetic energy is realized as force, and is distributed evenly across the fist. (This is what they say)