It depends on your item, team and format.
If it's doubles, go for Stone Edge. Rarely will you get the chance to set up in doubles. Rock Slide would be better too because it'd hit both your opponents.
If it's singles, it depends on your team. If you want Garchomp to be a revenge Pokemon, give it Scarf and Stone Edge (also suggest Iron Head > Outrage). If you want your Garchomp to sweep, give it a Focus Sash and Swords Dance; in this instance I would replace Outrage rather than Stone Edge.
