Neither, in my opinion.
Your average Dragonite has the moveset formula of Dragon Dance / Dragon Claw / Extreme Speed / Fire Punch, letting it not be completely walled by Steel types. However, out of the two moves you put up, I would opt for Earthquake.
This lets you nail Heatran and Ferrothron, who otherwise wall the heck out of Dragonite. Iron Head looks good on paper as a check to Ice- and Fairy-types, but in practice it usually doesn't perform as well as Earthquake (or Fire Punch).
Iron Head would be a good option if your team suffers heavily from Fairies and Ice-types, but otherwise use Earthquake.