Both have meaning "save". In the second expression, "쓰다" means "use", and it is proper for a tool supposed to be used. 1. 물을/를 아끼다: save water(물) 2. 연필을/를 아껴(서) 쓰다: save a pencil(연필) "아끼다" also has a meaning "value, prize, cherish, or hold <a thing>" 어린아이를 아끼다. (cherish a child)