The spring is horizontal and therefore the force applied is equal to the Tension in the spring.The work done by external force is equal to the elastic potential energy stored.
When the spring is vertical , the force applied is not equal to the Tension in the spring as weight is in the same direction as the force applied.The work done by external force is less that the elastic potential energy stored in the spring. There is epe stored in the spring due to the mass hanging on the spring.
