First let's take a look at both moves.
Iron Head -
Base 80 power
Has 30%chance to flinch
Base 100 power
Has 30% chance to lower Defense
It may seem like Iron Tail is better but Iron head is 'superior' to Iron Tail for one main reason:
Accuracy is important because if you are In a close match against an Ice-/Fairy-type, Iron Head can get you the win because it is accurate. Iron Tail is not accurate hence you may lose.
Hence here Iron Head is the better move for your Garchomp.
Source for Iron Tail
Source for Iron Head