Oh forgot to mention Gyrados has bad Sp Atk and most water moves re Special, Aqua tail is the only move i can remember that is a physical water move so i would get milotic
Actually, strategy-wise Gyarados is the worst choice between the two due to its typing. Milotic only possess two x2 weaknesses, and has good defenses. Gyarados' only advantage is its ability Intimidate, and even that is only circumstantial. I choose Milotic. :3
It's my job to help the next generation, and set a good example for them. And for that I'll gladly lay down my life.