Depend how much SP do you have and what lvl are you :)
Diamond Dust - from practice: good powerful AoE skill.
Great at exp in FoG or Dino Crater (dino in crater almost/actually one hit)
Additionally while it got 40 % speed reduction is quite good in mass PvP when you step too far and you got melee on back.
Throne of Ice - haven't got it yet - lack of SP after getting Bubble (have at PTS but not use too much so far so I'm not best source ) :)
But if you are 79 take crusher and wait for 80 to get Bubble (Raging Waves).
It is very useful in all instances and precious in mass PvP.