Frost Breath - 60 BP 90 Acc
Always results in a critical hit.
That means it's Base Power is always 90BP, and it ignores stat changes. (SpD boost)
Ice Beam - 90 BP
10% chance to freeze the target.
Has the same amount of BP as Frost Breath, but can Freeze your opponent, which can be game changing, and it has 100 Acc.
So overall, they have the same "power" but different uses: if you need a set-up (Calm Mind) breaker, go with Frost Breath, if you want reliabilty and maybe a Freeze, go with Ice Beam.