Graduated filters are available as either capturing has a steep change in light or brightness from one portion of the image to another. Soft edge filters are used when there isn't a definite or clear distinction between one section of the image and another. It's smoother so it allows for a natural transition of light. Take for example the difference between an image of the countryside versus an image of a city skyline. Countryside photos are flat so there is a definite line separating the bright sky from the dark ground. For those sort of images you want a hard filter. A city skyline has skyscrapers that pierce into the sky so you want to use a soft filter to compensate for where the sky is breaking through the buildings. One thing you should acknowledge before making your final decision to purchase a filter is that each brand differs in its definition of hard and soft edges. One brand's soft edge could be equally as harsh and abrupt as another brands hard edge. Look at the darkness of the filter itself and determine if you think it works for your purposes by sight.
