Frost has quicker AoE, you can use Fury and reach your ceiling, same with other abilities very low end ramp up time.
Unholy needs to utilize DnD, dot, use DT and after that it's a cycle between SS/CS and DC to maximize your aoe damage. The lower the mythic dungeon the higher a frost will pull numbers compared to unholy and the opposite is true also.
The only reason you would prefer frost is due to the aoe stun in M+, an utility unh lacks.