Category 8 is slightly larger than category 9. Some category 8 have an additional Murphy bed directly in front of the window, but not all. If that is important, you should check with room reports on specific rooms. Category 8 also have a round tub in the bathroom with a rain shower head. The category 8 portholes are also larger than the category 9 ones.
The exception to the above are category 8A. They are far bigger, have two portholes, but no split bathroom.
The letters behind the numbers indicate location on the ship. Category 9D and 9C are obstructed view portholes. As far as I know, they are all in the very front and the wall is at an angle. This gives you slightly more floor space. I would recommend checking room reports on these as some seem to be better and bigger than others. 9B is on deck 2, 9A on higher decks. With 8D to 8B it is just the decks that make the difference.
All forward staterooms are close to the theatre and the adult pool, aft would be closer to restau rants and Cabana. I think for a family with kids, aft would be better than forward.
As to which deck is good, I think everyone has their own preferences. A lot of people like decks that have no public areas above or below. For kids deck 5 might be good as it is the same floor as the kids clubs. Deck 2 is usually considered to be good if you want less movement. Deck 2 9B are also the only midship ocean view rooms.
