If someone knowsthey have a problem with seasickness the lower decks are better. If you've never been seasick, carsick, airsick, etc., it really doesn't matter !
"A truly happy person is one who can enjoy the scenery on a detour."
A middle deck in my opinion, as its half way between everything. Also it does not get the same degree of sway the top ones do, and the turbulance the bottom ones tend to get. Also middle middle, ie mid in decks and mid of the ship as its the most stable place to be in my opinion