Softwood tends to be a lower density than hardwood (i.e. the same size log will be lighter). Because of this, softwood burns faster and is easier to light. So if you are looking for a fire that is easy to start and burns quickly, softwood is your best bet. However, hardwood burns hotter and for longer – relying on cheaper softwood is a false economy but it is useful to get the fire started.
You can also mix it with unseasoned hardwood to keep your fire going.