Slow and Powerful any time. Cause in real life, if two opponents are both good, the bigger one almost always wins.
Also in gaming, being able to oneshot opponents with a slow swing is always more efficient than having to hit them multiple times, even if you could kill them equally fast(usually because the bigger weapon has longer reach, and you can time the attack to hit just as you move in range). In addition, in most systems a strong attack is much more likely to stagger an opponent that you cant instakill. The flipside ofcourse is that a slow and heavy style requires a more patient, methodical approach, as if you are impatient and miss, you die. But that's part of the fun.
I would argue that generally the slow and powerful approach requires far more precision and skill in most games. Skyrim, FPS, Daemon/Dark Souls etc.