Overall, the main difference between hard and soft evidence is that hard evidence is always preferable to softer alternatives, for the simple fact that even the best soft evidence cannot outweigh hard evidence in any form. That said, there is still value to soft evidence, especially in the absence of hard evidence.
Soft evidence can refer to anything from simple word of mouth or argumentation, to authoritative opinion on a given subject. The highest form of soft evidence tends to be supporting opinions from an authority with certifiable credentials. While random guessing also qualifies as soft evidence, it is generally not worth pursuing given that even the highest form of soft evidence are still just that: soft.
Soft evidence does still have value, but only in the absence of hard evidence. If there is no supporting hard evidence, then the best available forms of soft evidence should be used in the meantime.
A further problem with soft evidence is the appeal to authority. While most of the time an authoritative opinion will be valid, it is not always the case. There is a danger of placing too much faith in an authoritative source, as it is important to remember that even though it is the highest form of soft evidence, it still does not outweigh hard evidence.