Warwick E vs. Gragas W

Which one is best? Both include damage reduction for 2.5 seconds.

Gragas W: Drunken Rage COST: 30 Mana COOLDOWN: 8 / 7 / 6 / 5 / 4 Gragas reduces incoming damage for 2.5 seconds DAMAGE REDUCTION: 10 / 12 / 14 / 16 / 18% Gragas' next basic attack within 5 seconds deals bonus magic damage BONUS MAGIC DAMAGE: 20 / 50 / 80 / 110 / 140 (+ 8% of target's maximum health) (+ 30% AP)

Warwick E Primal Howl COST: 40 Mana COOLDOWN: 15/14/13/12/11
Warwick reduces incoming damage for 2.5 seconds DAMAGE REDUCTION: 35/40/45/50/55% At the end of the duration, or if activated again, Warwick howls, causing all nearby enemies to flee for 1 second.