I can't really say which one is best for the encounter. I believe it is said in the arcane PVE guide although I may be mistaken, but the difference between the mastery and haste build is very small.
I've recently only killed heroic Elegon once, and I personally think that the mastery build would be better for this fight. My reasoning for this would be simply that with the mastery build your spells hit harder. With a good critical hit I can nearly one shot the Energy Charges (orbs), which might give me more time on the boss because I am not spending additional casts trying to kill the orbs. Then again I have not used the haste build for this fight, I just like watching the orb I am assigned to drop like a rock.
Oh and just something in case the group decides to put you on the Total Annihilation rotation, Greater Invisibility is your friend.