Everyone has a different opinion, but I prefer an inside cabin down low and in the center of the ship. You will feel a minimal amount of roll and pitch in this location and sleeping in an inside cabin is much more satisfying.
Just my opinion, so don't flame me for it.
Also, if motion sickness is a possibility, I would strongly suggest taking one of Royal Caribbean's Voyager class ships. These ships are so large that they are rarely affected by movement.
Have a great cruise!