I'm afraid I can't sugar-coat the answer at all: Altaria is not a threat; not even in its tier. Dragonite is a better choice in every aspect. Altaria seems like a decent Pokemon, thanks to the nice amount of bulk it has, but the fact that the only way it can abuse the bulk is with Toxic makes it extremely unorthodox and, dare I say it, useless.
Altaria actually has a disease that a lot of people have named after Dragonite, your other choice. It's called "Dragonite Syndrome", and it describes a Pokemon that became no weaker, but because its competition got so much stronger, it's just overshadowed now. This is usually what happens to Dragon-types, Steel-types, and legendaries, seeing as their fast-changing rosters usually mean that certain Pokemon get left behind in terms of additions between generations (Mawile, Flygon, Ho-Oh, etc).
Falling back on your Altaria or Dragonite question, I'd have to say that Dragonite is a much better choice. Having a 600 base stat total means that it's really strong all-around, having a massive Attack stat, good bulk, and even an okay Speed stat. Altaria really just has Defense, and it would take two Dragon Dances just for Altaria to be as strong as a regular Dragonite. Pair that crappy Attack with a mediocre movepool, no form of abusing the bulk, and less HP than desired, and you have a really mediocre Dragon.
Definitely take Dragonite. You won't regret it.
