Given that the reason Gandalf wanted Smaug out of the way was to prevent his "use" (I think that's the word used in the books) by Sauron, I always figured Sauron and his forces were much more powerful than the dragon.
Not necessarily in terms of physical or combative ability, but just due to the fact that Smaug is a "dark" creature, and it seems like all the "dark" creatures of Middle Earth had come under Sauron's sway as his power grew. Even if Sauron didn't directly control them, they tended to do things that benefited him.
I imagine that had Smaug not been killed, he would have woken up as Sauron got closer to launching his assaults on Gondor, etc., and fucked shit up around Erebor. Smaug might assume he was doing this of his own free will, but Sauron's influence probably would have been the catalyst that started his rampage rather than staying asleep on his piles of treasure.
As to the Nine, weren't they pretty much unkillable (save for the one prophecy) due to being linked to Sauron himself? Smaug might be able to repel them for a while or disable their mounts, but eventually they would kill him.