we've stayed on deck 6 and deck 8, and they both had great things about them.
Deck 6 is a little more economical and fairly centred in the ship.
Deck 8, the cabin is a bit bigger and it certainly helped with my exercising (I vowed to never take the elevator anywhere... so it was great that I only had to walk down one flight of stairs from the pool when I'm wet, but coming from the other decks, I had to hoof it).
I personally found no difference between the decks for sea-sickeness (and I get sea-sick let me tell you!). My only suggestion would be to book a cabin fairly centered (as opposed to the ends of the ship)
Also, a trick my dad taught me (he was a sailor in the Canadian Navy) is to take bonamine three days before you actually get on the ship. It really does help (I didn't try it on our first cruise, but I did on the second and I noticed a remarkable improvement). Also, I found the Western Cruise to be a lot smoother sailing than the Eastern.
Last tip, even if you're nauseous, eat something and try not to drink as much. Obviously, don't let yourself get dehydrated, but cut down on the water, and make sure you have something in your stomach (preferably a carb of some sort to soak up some of that acid).
Hope this helps

Katdb