IMO, I'd choose the Riviera deck over the Main deck. The Riviera is under other cabins, the Main is under restaurants and the galley (NOISY
I would price a 6B on Riviera deck. (I tried, but the web site seems to be having problems-big surprise
As mentioned, the lower you are and the more midship you are the less movement you'll feel. Think of a seesaw-the middle has less movement than the ends.
Have a great cruise wherever you end up!