I'd have to agree with Sandy-only you can decide if the extra money is worth the balcony. We would not have an interior cabin simply because we enjoy the fresh air, the views from the balcony, and even having the sliding door open while sleeping. If you can swing the cost, my suggestion would be go for the balcony-you will NOT be sorry.
As an aside, even if you have mild sea-sickness problems, go for a cabin in the middle of the ship. The motion is minimal, and even the view doesn't move as much.
Good Luck and Enjoy-you'll be hooked like the rest of us here!