Ok so here is the comparison. First off, abilities.
So Dragonite has a decent ability Inner Focus, this can be handy for use ingame agaist those few pesky Fake Out users. He also has the dreamworld ability Multiscale. This is insanely useful in the metagame because it allows him to survive many would be KOs!
Salamence has the great ability Intimidate! This is sop useful against any physical attacker both in the metagame and ingame. His dreamworld ability is absolutely outstanding! Moxie!!! After you have KOed on you get a free +1 in attack making you a more and more dangerous sweeper and an even more dangerous Choice Scarf Revenge Killer.
Next Up, Stats.
Dragonite has a rather low HP stat at 91, but that is made up for in is exceptional defenses. His 95 Base Defense and his 100 Base Special Defense make him a rather bulky sweeper. His speed is rather lacking, but his true niche come out with his attack stat. 134 Base Attack is awesome! And his 100 base SpA is also a decent stat.
Salamence sports an amazing 135 Attack stat and a great 100 Speed stat. Unfortunely, his defenses are weaker than dragonites which does not allow him to stay as long nor become a bulky sweeper. And he can also go mixed with his 110 base SpA stat.
Movepools!
Dragonite wins this by a mile! It may seem that they have almost the same movepools but Dragonite's is better in a few neat ways. First he has access to Hurricane, his most powerful STAB flying move. This is great because he is so often used on rain teams sporting both Thunder and Hurricane. He can use may status move because of his Multiscale ability. He also has access to Super Power, the Elemental Punshes, and ExtremeSpeed!
Salamence's movepool may seem huge but he is actually rather limited. His mediocre defenses do not allow him to do much excpt attack causing him to not be able to use a good chunk of his moves.
Sets...
Dragonite:
Here are two super useful sets for Dragonite.
Dragonite @ Life Orb
Multiscale
Adamant
252 Atk/ 4 Def/ 252 Spe
Outrage
Fire Punch
ExtremeSpeed
Aqua Tail
Dragonite (Rain)@ Leftovers
Multiscale
Modest
252 HP/ 4Def/ 252 SpA
Hurricane
Thunder
Substitute
Roost
Salamence:
Salamence @ Choice Scarf
Moxie
Naive
252 Atk/ 4 SpA/ 252 Spe
FireBlast
Dragon Claw
Outrage
Brick Break
Salamence @ Life Orb
Intimidate
Adamant
252 Atk/ 4 Def/ 252 Spe
Dragon Dance
Outrage
Earthquake
Brick Break
Thats all folks! I hope you can now choose which is best. In my opinion, they are equal(except I like Dragonite more :D)