Arcane and Holy...

Reasons?

Because the AOE damage from both Arcane Explosion and Holy Nova = great.
I leveled really fast with 4 Mages and a Priest.

And with the healing tree, I at some point stopped a little after I got the Talent HN, but I guess they changed that now and its a vendor spell. So you might could get some decent healing talents, then jump over to Dis for the longevity?

The reason for Arcane is because of the mana saving talents/crit talents/ all around, a decent tree. And since the only thing you are going to be doing is running into a mass of mobs and AOE the piss out of everything. Just cast Frost armor/Power Word: Shield on the Priest, and run a muck!