Single handed slashes and strikes seem, to me, to be more orriented for speed and more versitile movement. With one hand on the blade, you can move it more freely provided you have mastered the weight of the blade.
Two handed slashes seem to be for more power based, hacking threw flesh, breaking armor or one's defense. Reason would be due to the reinforcement of the blade with the second hand. Using both hands you have more power than you would with just one hand. I suppose, since you could move the blade with two hands just as well as with one hand, you'd get a sizeable amount of speed as well, however when I think of two handed anything, I feel it's more focused and generates more power