I'm gonna say Sandstorm
Why? Garchomp would run fire fang to cover steel types neutral or immune to it's ground STAB, such as Bronzong, Skarmory, Forretress and Ferrothorn. Although these Pokemon are quite common in the competitive environment, they are very rare in game, to the point you won't even see the latter two outside the battle subway. Sandstorm is very handy to Garchomp, as his evasion will be raised and damage opponents that are not rock, ground, or steel types. Stone edge, eathquake and outrage all hit ice types harder. So, if this for in game, go for sandstorm, but if you find that steel types are giving you trouble in wifi battles or the battle subway, give fire fang a try.
Hope this helps