I know most people find that the lower you are, the less movement you feel. For someone like myself (I have terrible stomach problems), decks don't make much of a difference.
One hint if you suffer from motion sickness (a trick my Dad, a navy sailor taught me) is to start taking bonnie (or some anti-seasickness med) a couple of days BEFORE you actually get on the ship.
I forgot to do this on the first cruise and paid dearly for it (Christmas cruise, huge waves, and the waiter kept telling me "Don't look at the christmas trees" that was swaying on the ship as if caught in a hurricane... yeah right!)
On our second cruise, I took bonnie the three days before the cruise, and I was much better. Also, the first night generally seems to be the worse (on the Eastern) as you are accomodating yourself to a moving ship AND going through the gulf stream! So don't get overly discouraged if you do feel a little queasy the first night, it normally gets better!
Hope this helps
Katdb