If your prone to motion/sea sickness being on a lower level and towards the middle of the ship is best. Personally, I prefer to be higher up and towards the back of the ship. There is less traffic at the back since only passengers with cabins back there head that way, usually. I like being higher up since its usually one flight of stairs up to the pool and Windjammer (of course it all depends on the ship too). I also like being higher since thats where the balcony cabins are and the view is a bit better. Just my opinion though.
On our first cruise, we were on the fourth deck, outside, almost all the way to the bow. We ran into 27 Ft. seas, and could really feel the ship rock and roll. There was a big difference from there to midship. I'm just glad we weren't up higher could have been pretty bad. We have been on the eight, and the ninth deck too with smooth seas, and really enjoyed the view from there, but from the seasick point of view, lower and toward the middle is best.