Neither. Use Ice Beam instead. Since this is a play through, I will go only by base stats. Froslass has 80 base Attack and Special Attack, so the difference between special and physical attacks are negligible. Now, since Frost Breath always results in a critical hit, it will have a base power of 90, or in Gen. 5, it has a base power of 40, so the critical hit will have 80 base power. So, unless it is Gen. 5, it will have the same as Ice Beam, unless you are playing in Gen. 5 or 4. In that case, it has a base power of 95, so it is stronger. Ice Punch only has a base power of 75, so it is weaker. The added bonus of Ice Beam is that it gets a 10% chance of freezing the target, so that makes it a better choice overall than both Frost Breath and Ice Punch. The only downfall is that it is a TM/TR that is usually obtained later in-game.