I can't speak to the deck 6 cabin, but I've stayed on deck 7 aft twice. Once in 7106, once in 7624. I thought both locations were great. Very quiet, with quick access to the aft stairs and elevators, plus the "secret" verandah at the stern on deck 7. 7123 would be right across the hall from 7624, except on the port side of the ship. I thought it was a great location.
Two of the three restaurants are also aft, so it makes going to dinner very convenient on 2 of 3 nights.