They launched the fireworks last year simultaneously from two of the coves: the cove in front of Aulani (on the left side as you face the Ocean), and three or four coves down on the left in front if the Marriott times shares.
We watched from our 15th floor balcony (ocean facing, over looking the area they show movies and have the starlight hui). When we checked into out room, I was disappointed that we were looking at the wedding chapel for our view, but that opinion did a 180 when the fireworks went off - we could see both. I will post a picture when I get a chance.
Other than a view like we had, I would say you want to be as far toward the ocean as you can get, and probably toward the JW Marriott if you want see both at the same time from ground level.