April 26, 2015 9:36 PM EDT
That's a tough one.
In one aspect, death is cold, much like a corpse is cold; while fire is the warmth of life.
In another aspect, cold numbs and puts to sleep, while fire causes searing pain and agony.
So I guess it really depends on what aspect of death you're going for. If you're shooting for a corpse / sleep aspect, I would say Frost. If you're going for the pain/agony/burning in a lake of hellfire, go fire.