"Damage over time dice?"

[Jiipster]

And?

Your crit rate is the same for both spells which means that the average increase in spell damage is the same for both spells. Twice as many crits for half the damage works out to the same amount of added damage. Unless you have some kind of effect that produces a bonus whenever you crit with a spell, there is absolutely no advantage for Icestorm to produce more crits.

Non-epic trash will die faster, since your damage output will be more frontloaded.