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This depends on which catagory cabin you choose. I have been on 19 cruises - 2 with P&O, 16 with Princess and 1 with Celebrity. On Princess and P&O which ever catagory you coose, my suggestion is to pay for the cheapest of that catagory due to the fact that all the cabins are more or less identicle. You are simply paying the extra for the position on the ship. You will find the higher up the ship, and if you choose a mid ship cabin you will pay more. If you are wanting an inside cabin, when you are inside that cabin you are unaware how high you are positioned on the ship, so go for the cheapest. However midships is very likey the more stable for movement should the sea be a little rough. Princess in my mind is an excellent choice, whichever ship you choose to go on from their fleet. If you can afford a balcony cabin midships - go for it. It would be an excellent choice.
Usually the higher the deck the more expensive it becomes.