Category 8 is divisible by 2. Category 9 is divisible by 3.
Other than that and the deck, I think they are the same. I upgraded to Cat. 8 after I found out Cat. 9 was at the bottom. I just like being up higher. I'm told that on Disney ships it isn't like this, but on a RC cruise the lower decks seemed to have smaller hallways and felt more like the bottom of the barrel to me (though I never saw the cabins).
Scott