I would recommend the balcony for a couple reasons 1) the tiny bit of extra room 2) you have a good place to view the Na Pali coast on the last full day as you are leaving Kauai.
we have done three adults in a balcony room on three different NCL ships plus a child. It was tight but manageable assuming that everyone gets along.
I would say that the Hawaii cruise, while it was just my wife and I in the cabin, was the least amount we have ever been in a cabin on a cruise because the shore excursions tended to be all day event.
Whenever it is three adults in the cabin, one of the things that we would do is that one or two of us would get ready for dinner in the spa or fitness locker room.