21 Blast or Stun Mastery

I think that getting Blast to 21 would be much more useful, for, not only does it hit an extra target, but it does significantly more damage. Like others before me have said, Stun Mastery requires using DU or BSB to activate the initial stun, and the it only adds 60% damage onto your attack when maxed. Even if a mob of 4 wasn't present, the extra damage from Blast should prove better than Stun Mastery's.