The sand dragon. If you don't care about cheapness Double Team Garchomp in sand is a nightmare, but without it's still a great Pokemon, sand or without. High Attack and Speed, definitely outspeeds Dragonite. Has natural bulk, and can take anything par Ice Beam and co... But yeah. A great Pokemon for a naturally fast coverage sweeper. Has a great movepool, being able to learn move types to cover every Pokemon for super-effective or neutral hits. So yeah, hits VERY hard. Great pokemon for in-game.
The first dragon. Pretty bad speed, but bulkier than Garchomp, at least in terms of being able to take Ice Beams :P
Great ability come Gen V, Multiscale rocks. High Attack, very high attack, but mediocre speed. Makes for a pwoerful Choice Scarf Outrager. Also great at DDing, then sweeping maniaclly. So yeah, also very good. Hits hard if you don't care about taking hits. Also an amazing Hurricane abuser...
So yes, both good. If you want a fast (and, if you want, cheap!) coverage sweeper, then choose Garchomp. If you want a setup, yet ridiculously powerful Pokemon that's a bit more risky, then take Dragonite.
I hope I helped!