My wife is the same way - some people feel motion more. Motion is definitely less mid-ships, and it is much worse far forward than it is partially forward. You want to stay away from the extremes.
Lower decks are slightly smoother than uppers, but with stablizers (which all ships have now) the side to side motion you might feel more on upper decks is rarely a factor.
Having a balcony is a really good thing for people who get motion sickness - they get fresh air and can see the horizon - this often helps. Most people prefer them.