Well apart from the obvious number of hands difference, a one handed slash diffsers from a two-handed one in the areas of power and 'flexibility'.a two handed slash offers more power,and hence destructive force but with little ability to react to change after you launch this type of attack.
The one handed slash however,being deficient of the same power and strength of a two handed slash serves as a less deadlier attack but providing the avenue to quickly react even while slashing.You are granted the space to change the direction of an attack an may even decide to stop the attack completely..