While re-reading the book recently, my initial thought was that it would end as they are leaving Laketown, as that felt like a good stopping point at the time... and then I remembered that the second film is called The Desolation of Smaug... so they HAVE to go farther than that.
I also started thinking about Smaug himself. Although he is built up as the main villain, he dies well before the end of the story... so it seems like he either has to die at the end of the second movie, or early in the third. The former seems better, as it would build up to a good ending to the second film. If it occurs early in the third, it will be somewhat of an after thought.
At e same time, how then do they fill in the third movie? I'm sure they will greatly build up the battle of the Five Armies (it is the title of the film, after all!), and perhaps the Necromancer stuff will play into that film moreso than the 2nd... but at the sae time, he has to e dealt with prior to the battle since Gandalf must return for it.
If not for the title of the second film, I might guess that the Necromancer stuff would be more prominent in the second film... I can't see both Smaug and the Necromancer being dealt with in the same film though...
My current guess is that it ends with Smaug dead, but the Dwarves not knowing it yet.