The older ships have a different hull design the more modern ships have stabilizers which remove most of the roll in heavy seas,but are more vulnerable when the wind gets up due to being several floors higher.As the ships are taller you will feel any movement more on the higher decks so the best cabin would be center low down the higher or nearer the front or back you get more movement,the extra weight will help but personally feel that you get more roll on the bigger ships its like driving a big limo or an Escort the ride is different?
The answer is a bit complicated, a lot depends on the ship design and the nice techy stuff they install to stop the ship rolling about. As a rule heavy is good, it's more weight to throw about and the ship will ride through most bad seas in relative comfort. It also depends on the angle of the bad weather, over the bow and stabilisers can correct that, over the stern is similar but a port/starboard hit of bad weather can make a roll. The bigger/heavier the ship the lower it sits, and the better the ride. I was suprised when I was on Indy in June, we had a heavy swell (force 10 the telly said) in the Bay of Biscay and we basked in comfort on deck 2. I also noticed the very low incidence of sea sickness during this weather, i have seen more on the Dover Calais ferries.
Bon voyage !!