The most comfortable position on a ship in the event of heavy seas, is most definitely the middle of the ship. The front of the ship would incur more movement than anywhere else.
Definitely go for the middle of Deck 6. I have to be careful where I choose a cabin as I too suffer from travel sickness and before my first cruise in 2003 I took the advice of the Rep. in our local Travel Agency and booked the middle of the ship both deck level and cabin location and was fine.
We always go for the middle of the ship if possible. otherwise go for the back end. it doesnt move as much as the fron.
If you get sea sick I should head for the nearest toilet, be it at the front , back or in the middl.
My friend was always sea sick until we took her on a P&O and booked an inside cabin. She now travels all the time but only with an inside(no ocean views) I am however told by people that work on these ships that the best place for people with sea sickness is on the 2nd or 3rd floor - middle of ship. Better lower down.
If you get seasick the best place to be is as low down as possible and in the middle of the ship.. out of your options, I think I would go for the middle of deck 6, even though it is a little higher, you are still going to be better off in the middle of the ship.