Soft pastels contain less binder which gives them their velvety texture or "bloom". They contain more pigment, so the color is rich and can easily be blended or smudged with a finger or soft tool. The down side is that they are more delicate and can break easily. The same softness that makes them easy to blend also allows them to be accidentally smudged, so it's a good idea to use a spray fixative once you complete your work. Their soft texture creates more dust which can be a problem for some people.