if you are going to use a shaman class, i would consider a mystic because of their group slow and their group debuffs. A defiler has single target slows.

Although a defiler has a 10k ward and they have 2 Hp buffs while a mystic has bolster and 1 hp buff and a stanima buff. ( i find the hp difference is about 100 different)

A defiler does more dps with his debuffs though as well so if you want extra dps then that might be the way to go.