Which deck is best is definitely a matter of personal preference. The category 6 rooms are on decks 5 and 6. The category 5 rooms are on deck 7. The only difference between these rooms is the deck. I personally like to be higher up on the ship, but I've read several people stating that they like to be closer to the water. Another consideration is that there are more rooms on deck 6 than on any other deck on the ship, so there may be more hallway traffic and noise. I've never stayed on this deck, so I couldn't say whether this is really a problem or not.
Another verandah idea is category 7, if your party has 3 or less. These are the semi-enclosed verandahs. I actually prefer these, and my first choice is a cat. 7 on deck 7.
Also, any category 5 or 6 room which is aft of the category 7 rooms will have a solid metal railing, rather than the see-through plexiglass railings which most of these rooms have. On deck 5, these are rooms 5142/5642 and aft; on deck 6, rooms 6144/6644 and aft; and on deck 7, rooms 7130/7630 and aft.