I think it is kind of hard to compare rocking on decks unless you experience identical sea conditions.
When we cruised a year ago we had seas 20 to 30 feet. We were on deck 6. We really did not see much of a difference between our stateroom rocking and say deck 3 rocking. They both rocked! (in a bad way). I did not actually get seasick, but my poor husband did.
We are booked in a CAT 9, 2nd deck for the next cruise. I wanted to stay lower to minimize the rocking effect, but my husband feels that it really didn't matter what deck we were on, on that cruise, we felt it everywhere.