They are both good abilities. Which one is better?
Its a matter of opinion and can also revolve around the format of the battle and your team. Water Absorb can be useful in doubles but is very situational in singles. I prefer Unaware as you can stall out alot of teams that rely on stat boosts. Here is a set that you might want to use:
Quagsire @ Leftovers
EVs: 252 HP / 252 Def / 4 SpA
Quagsire can have these abilities:
Water Absorb - Raises HP when hit by Water-type move
Damp - Prevents the use of self-destructing moves (Also prevents damage from the Aftermath ability)
(HA) Unaware - Ignores any stat changes in the Pokémon
I personally pick the Damp.