Are you BOTH Two-Boxing or did I read that wrong? You're both going to group together? As for your question, I would say Fire - It's the best for maximum damage and AoE (Dragon's Breath). Ice is a possibility but there's a lot of stuff in that tree that needs manual mouse clicking like Blizzard / Pet Frost Nova and whatnot to get a lot out of it. Fire is just PEW PEW.