The difference is that the Extreme Impact has added technology called shock pads built into the bottom of the mouthguard, and the Extreme Impact comes with a mouthguard case.
The High Impact does not come with shock pads nor does it come with a mouthguard case.
High Impact Mouthguards and Extreme Impact Mouthguards both come with sweet designs and both mouth guards are made from our most shock absorbing material Damprotech™ which has been tested in multiple studies to be more shock absorbent than the most common material used in mouth guards.