Downward

Aren't power strikes of various types more powerful "downward" like the difference between breaking a board suspended by a string or thrown, than braced on both ends for breaking. Likewise aren't the blows where opponent is on the ground under them more telling than that.

I don't use "power" strikes per say. If I were, I assume downward might be better since gravity works with you.

I rely on speed, not strength or direction.

While my techniques are crisp and controlled, I am looking to go through whatever I hit. I train for power, precision, and speed and I train hard and bitter. I think that your two most powerful weapons are downward strikes, The downward knifehand, and the ax kick. Howevever the fact that they must first go up means that they take longer and are more predictable. So then they arent your strongest or most powerful techniques are they? What good is the strike if it gets stuffed? Your most powerful techniques doesnt always mean the ones with the most physical impact. On a purely physical level however yes downward strikes are stronger.