Let's compare Salamence and Dragonite first:
Salamence is the more offensive one, holding a base 135 physical attack stat, and a base 100 speed stat. It also has the potential to be a special attacker with its 110 special attack stat (although this really isn't favourable for Salamence.) Its abilities, however, are where Salamence has the upper hand any day. Having Moxie as a hidden ability allows it to set up 1 Dragon Dance and then go for a sweep while getting stronger with each KO. (again, this is why Salamence sets shouldn't be special attackers, unless they're going for the Intimidate ability that Salamence has.) Also, Salamence has a mega evolution, boosting its sweeping potential even higher and giving it another ability; Aerilate. Now, Aerilate coupled with Double Edge, Thrash, Giga Impact, Hyper Beam etc. gives any normal move STAB and boosts its power by 30% (I think). So, in other words, STAB Aerilate Hyper Beam has a approximate power of near 290 (if STAB is applied first) and a power of approximately 270 (if Aerilate is applied first). However, using Mega Evolution does mean that Salamence loses its ability to hold an item, so no Life Orb or Z Crystal strategies if you're planning on using Mega Salamence. (If you're playing by Smogon rules and not VGC, Mega Salamence is OU tier, while normal Salamence is BL tier.)
Dragonite is the more bulky one, with 91 base HP, 95 base Defense and 100 base Special Defense. It does have an astounding 134 base Attack stat, but its 80 base Speed stat is a bit lacking. This means that Dragonite does have the potential to Dragon Dance sweep, but it doesn't sweep as well as Salamence or Mega Salamence. Dragonite's 2 abilities (Inner Focus and Multiscale) are useful in some situations, but Multiscale is probably the more useful one as it allows Dragonite to live an Ice Beam and get a Dragon Dance off, then proceed to sweep the team. However, Dragonite is meant to be bulky, and while you could argue that Salamence can't live an Ice Beam from a decent Special Attacker, but who needs bulk when you can outspeed most of the Pokemon in your respective tier. ( Although STAB Ice Shard is a problem to Salamence).
So, overall, Salamence is better because it has more potential to sweep teams, and can also stop Physical Attackers in their tracks with Intimidate, and proceed to Mega Evolve, Dragon Dance, then sweep.