I'm not going to make a long explanation as I'm pretty sure any and every major point has already been posted by an earlier user.
Both Pokemon are great, however, Salamence is much easier to use than Dragonite.
They way I look at it, Salamence is easily the more sweeping-oriented of the two. Now someone tell me, is it really that difficult to use a sweeper with that Atk/SpA and Speed? This reason alone probably makes him the more immediately appealing of the two (of course, this extends off into Choice sets, DD, etc).
In the right situation, however, Dragonite is probably the more dangerous, with an extremely unpredictable movepool and bulky stats. Once your opponent sees you're using a SpecsMence, Blissey (and Empoleon, to a lesser extent) become the first thought to mind when you need to handle it. Unlike Salamence, Dragonite can handle these switch-ins with Focus Punch, assuming you know that these Pokemon exist on your opponent's team.
Like I said, Dragonite is far more dangerous in the right situation, but finding that situation is rather difficult. Thankfully, luring out Blissey isn't a very difficult job.
Several people here are undermining Salamence's better Speed stat. That Speed is actually quite significant, as it means Salamence can outspeed more (making him more adept as a sweeper) and makes it actually worthwhile to invest a large amount of Speed EVs.
I guess I turned this into something decently long as I said I wouldn't. XP
In the end, however, I really think it comes down to:
Movepool/Defensive possibilites and Ability/Raw Sweeping ability.
It all comes down more to what your team specifically needs.