A hard night guard is proven to be the better option for most patients. The reason is that a soft night guard makes the mouth and muscles want to chew on it at night time as if it were a piece of gum. This chewing can leave the mouth sore the next morning. Hard night guards also generally last longer than soft night guards as hard ones can take more of a beating.
However, some patients do not find hard night guards comfortable and will not wear them. For these patients, soft night guards might make sense as the guard is still protecting the teeth. Generally, for these patients, we pay extra close attention for the first few weeks to make sure that the jaw and the jaw muscles are adapting well to the mouth guard.