It depends, they are basically the same, the main difference being their Abilities.
If you are going to pair it with Tyranitar or Hippowdon then it might benefit from Sand Veil more, but if you are not planning on having Sandstorm up then Rough Skin would be the better choice.
Their IVs are the same too so that won't change anything.
I won't compare moves, because apart from Iron Head there are no moves that one Garchomp can learn that the other can't.
So I think the better choice would be Garchomp 1, for its Ability Rough Skin, as it is a useful Ability in any situation, sandstorm or not.