This depends on: the type of photography, the type of equipment, and the requirements of an individual situation. There is no simple answer, most professionals will carry a selection of both. Hard Grads tend to be used on smaller format cameras (DSLR or 35mm) and are used for general landscape shots with a definite horizon. Soft Grads tend be used on larger format cameras or on very wide angle lenses and are used for balancing exposure evenly, when there is no definite horizon i.e. in cities or other situations when you may have an uneven skyline.