Soft coins are smooooothe. They don't "talk" as much when you rub them together. When coins talk its the clang sound you hear. Barbers are a really good example of a soft coin.
I think i can clarify. A soft coin is a coin that has been worn down a lot to make the face very smooth and "soft". This prevents them from making noises when the coins slide against each other. They also look pimpin
So, I take it then that in general soft coins are preferred when you're dealing with more than one coin? And regular coins are preferred if the trick needs better grip?
If you are doing a lot of stack work with coins, I.E multiple productions, then the soft coins are better because you can slide them off the stack without making much noise. As for grip I think it depends on the handler.