Members of our party that were susceptible to motion sickness were in 5506, which is very far forward on deck 5, and didn't notice much movement. We were in 5648, which is very far aft, and we didn't notice much movement.
I believe
DCL has superior engineering and or superior stabilizers, becuase we seldom notice any movement during any of our 4 previous cruises with them.
It's impossible to compare directly, but we were on the Carnival Ecstacy and the tv monitor with the bridge report showed slight to moderate seas, and we noticed considerable motion. On other DCL cruises, the bridge monitor showed moderate to heavy surf and we hardly noticed anything. Both ships were cruising at a speed of about 21 knots.
From a theoretical view, you would have the smallest possibility for motion if you were on a low numbered deck and if you were mid-ship. The low numbered deck would tend to minimize the left to right motion and the mid-ship would minimize the up and down motion.
Since I have a lot of confidence in the DCL engineering, I would take any cabin in the category I was interested in, just for the opportunity to cruise again.
hope that helps