They work quite differently.
Gryph-hounds have the special ability to retreat during the combat phase, after they've attacked. Ideally, this could be before the thing they're in combat with activates, so they don't get hurt at all.
Aetherwings, on the other hand, can only retreat the same as any regular unit: in the movement phase, if they're within 3" of an enemy unit. However, what their ability lets them do is then charge later that turn, where most units can't charge (or shoot) in a turn where they retreated.
So gryph-hounds: special get-out-of-combat. Aetherwings: special get-back-into-combat.
It's worth noting that Watchful Guardians can also be used to get Aetherwings into combat without charging, where there's Raptors nearby and the distance rolled is enough.