Percentage difference = |a − b| ÷ ((a + b) ÷ 2) × 100
Use percentage difference when you are comparing two values and neither one is the "starting" value, like two shops’ prices or two people’s heights. Because it divides by the average, it gives the same answer whichever way round you put the numbers.
Percentage difference between two numbers
For 50 and 75: difference 25, average 62.5, 25 ÷ 62.5 × 100 = 40%. Swap them and you still get 40%.
Percentage change is different
Percent change has a before and after: (new − old) ÷ old × 100. From 50 to 75 that is +50%, and from 75 to 50 it is −33%. Use change for prices over time, growth, discounts.
Which should I use?
A "before" and an "after"? Percent change. Two things side by side? Percentage difference. Comparing a measurement to a true value? Percent error.