If you are planning on using them competitively I suggest you choose on the basis of what your team needs. If this is for in game I would suggest Garchomp as it has stab Earthquake which hurts.
If you are planning on using the team over wifi I suggest on the basis of what your team needs. Garchomp and Dragonite are both naturally bulky (and if you can get multiscale Dragonite can take almost any Ice hit) but Garchomp has a very nice speed stat, therefore does not have to rely on Dragon Dancing to get its speed to decent levels. It also has Swords Dance which doubles it attack, though unfortunately does not get Dragon Dance which maybe would be more appreciated making it easy to revenge kill. Though Garchomp has one ability that Dragonite does not. Garchomp can set up stealth rocks which really help lands KO's.
That said I would pick Dragonite if you have access to its dream world ability because that means it can get 1-2 free Dragon Dances up before sweeping. It also has access to Extreme Speed which is very useful for landing KO's.
Overall if you don't have access to Multiscale, there is no reason to use Dragonite. For this reason I would use Garchomp as your attacker / rock setter.
Though if you do have multiscale then use Dragonite as it is great for sweeping and cleaning up kills with priority.
