Frost - Good mana effiency, can get inc. chill effect. Ice Block, Cold Snap and Ice Barrier are the best from the Frost tree.
Fire - Increased burst damage, not so good for mana. Blast Wave, Combustion and Dragon's Breath are the best from the Fire tree.
Arcane - Good support for the other two Trees. Clearcasting, Presence of Mind and Arcane Power are the best of the Arcane tree.
I'd spec Arcane/Frost or Fire.
But hey, thats me >.<
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