The two positions require different skills and abilities. A lead has to be superior to others at the tasks they do. A supervisor, however, has to understand the tasks the others do, but does not need to be the best at doing them. The key skill required of a supervisor is people skill, the ability to communicate, motivate, and lead. Too often we see production fall off because a company promoted its best producer to supervisor, only to find out that that person lacked supervisory talent and people skill. Management training can help, but know what you are doing when you promote.