I have a awasome Ferrothorn on pokemon white and just now it's learning Iron Head and i have Gyro ball with it. I know Gyro Ball is probably better in like wifi battles and stuff but it only has 5 pp and iron head has 10 and i nead pp to the Pokemon League so i'm not sure...
Ahem, I couldn't read that last one...
I say go with Gyro Ball because the PP isn't that big of a deal, and it will do much more damage, although you might want to consider Seed Bomb over Power Whip for your grass move.