Percent Change

Sand:

Trial 1 needed 550mL
Trial 2 needed 250mL

So the average between the two is 400 mL of water.
(400-100)/100 x 100 = 300%

Potting Soil:

Both trials needed 150mL

(150-100)/100 x 100 = 50%

The potting soil seems more prone to sliding since it requires less water to induce the mass movement and the percent change is smaller.