Okay. Here's the damage formula
Damage = ((((2 * Level / 5 + 2) * AttackStat * AttackPower / DefenseStat) / 50) + 2) * STAB * Weakness/Resistance * RandomNumber / 100
To find the answer you want, I'll assume a few constants.
Constants:
Level=100
Defense Stat=100
STAB=1.5
Weakness=1.0
resistance=1.0
Random Number=20.
Waterfall calculation:
Given---> attack Stat=243, attack power=80
Damage = ((((2 * 100/ 5 + 2) * AttackStat * AttackPower / DefenseStat) / 50) + 2) * STAB * Weakness/Resistance * RandomNumber / 100
Damage = (((42 * 243 * 80 / 100) / 50) + 2) * 1.5 * 20 / 100
Damage = 165.296* 1.5 * 20 / 100
Damage=49.48 or 50 with significant figures.
Damage Surf:
Given---> Attack power=95, SpAtk=200
Damage = ((((2 * Level / 5 + 2) * AttackStat * AttackPower / DefenseStat) / 50) + 2) * STAB * Weakness/Resistance * RandomNumber / 100
Damage = ((((2 * 100/ 5 + 2) * 200 * 95 / 100) / 50) + 2) * 1.5 * 20 / 100
Damage = (((42 * 200 * 95 / 100) / 50) + 2) * 1.5 * 20 / 100
Damage = (((42 * 200 * 95 / 100) / 50) + 2) * 1.5 * 20 / 100
Damage = 161.6 * 1.5 * 20 / 100
Damage=48.48= or 48. with significant figures.
-Waterfall does 50 points of damage.
-Surf does 48 points of damage.
there's not much difference, but you could always use stat boosters, EV training Etc.