You should get only what you need. The need depends on the situation. There is no "most commonly used ND/GND filter". Depending on the actual light conditions, the scene and the intended result (e.g. lowering the brightness of the sky) you can select from the range of GND filters.
Also, there are differences in the transition zone. "Soft" means that the transition from neutral to grey is broader and softer, compared to the shorter and harsher transition zone of the other filters.