Hi , welcome to the boards,
in a cat 9 you have an outside stateroom with porthole, you'll get some natural light and they sleep up to 4 ( so do cat 10,11 and 12) you get a split bathroom and all the facilities you have access to on the ship are the same .
disadvantages all are on deck 1 and 2 which aren't as convenient as higher decks, some people don't like to be on a low deck, some do , its'a personal choice. thats about it, there are some porthole rooms that have obstructed views if you need to have some sunlight , they are priced at cat 10 but are in demand and go quick they are room numbers 5020,5022,5024,5520,5522 and 5524 they are on deck 5 , you can check out pictures of these rooms at cast away club . com ( don't leave spaces and stick a
www. in front of it ) as i said these go quick but give you cat 9 at cat 10 price. If you can forgo the sunlight a popular deck is deck 7 category 10 aft, they have access to a small sunbathing deck that is still a bit of a secret. great for ordering room service and taking coffee and cake out in the mornings. p.s you get the same amount of room in a cat 10 as you do in a 9.
Happy cruising.