Gyro Ball, by far. Bronzong is slow, and is made for walling fast sweepers; so Gyro Ball will have high power. Flash Cannon is a lot less reliable, since Bronzong's Attack is higher than it's Special Attack.
If you are looking for power, Gyro Ball. PP doesn't matter much, really, since you have access to Elixirs and Ethers, as well as a Pokemon Center.