Answer
When sliding downhill, the overall net force is smaller.
Work Step by Step
When the box is sliding uphill, consider the forces parallel to the incline: the component of weight parallel to the incline, and the force of friction. These 2 forces both point downhill, causing the box to slow down.
When the box is sliding downhill, again consider the forces parallel to the incline: the component of weight parallel to the incline, and the force of friction. These 2 forces now point downhill and uphill, respectively, so their sum is smaller than before. With a smaller net force, the magnitude of the acceleration will be less.
