Data Value Change allows you to change the score to a specific number. For example, if everyone in a particular list should have 20 points, then you can change the value to 20. The Score change option allows you to add or subtract points from their score that already exists. For example, if you want to add 5 points to anyone's score who downloaded a white paper, then you would use the change score. If you used the Data Value Change for this, then you would be changing everyone's score to 5 and not adding 5 points to any score they may have already.
Hope that makes sense.