Both have their advantages and disadvantages. Imagine being on a slope, the above player has the luxury of forcing fall-damage upon the enemy, but may get hinged by being knocked-back into the slope just as if they're cornered. The below player also has a better range, seeing as the hit-box is full body but attack range comes from the head, and the top player also has the advantage of duking the opponent by leaping over them. If I were to be put into either of those situations I would choose the low ground, because from experience the range alone will give you the best advantage.