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Well with an inside cabin there does not appear to be many advantages. It has always been said that the higher up in the ship you are you are less likely to be sea sick but that might just be an old wife's tale and not being one who has that problem I can't verify it. Most of the attractions are around or close to Deck 6 so you will be nearer them with less need to use a lift or go up or down lots of stairs, but in the end how much are they looking for you to pay for the upgrade might be the final decider.
If you are thinking of upgrading i would think prob better to upgrade to outside cabin,stayed on deck 5 thomson destiny outside cabin,wonderful to wake up to different views every day.