The rooms themselves are identical, unless you're on Deck 1, which has the different portholes as someone mentioned. We've started booking only Oceanview rooms, and generally opt for the Cat 9 rooms on Deck 2. However I am somewhat picky about where my room is located so I only look at Cat 9 rooms in certain areas and will book Cat 8 if none of them are available. There were 2 recent situations when we booked a Cat 8 instead of Cat 9 -
1. January 4nt cruise on Wonder - Cat 8 was lowest available with Kids Free offer. We had room 7514, and while we loved the room, and the fact that the forward elevators were much less crowded than the midship or aft elevators, we found that we would rather be more aft and closer to the areas that we tend to go to with little kids, than being near all of the adult areas. We did a lot of walking to get to the restaurants, buffet restaurant, drink station and kids pool. It was nice though to be able to run quickly up to the room while waiting for the big shows to start.
2. Westbound Transatlantic in September - We've booked Cat 8 GTY for 2 reasons. The first was that at the time we booked, I didn't like any of the locations of the available Cat 9 rooms on Deck 2. The ones available were under or near kitchens or nightclubs. I also didn't want to book Cat 9 GTY when they came available because I didn't want to risk getting assigned a Cat 9 room in a location I didn't want. The other benefit of booking Cat 8 GTY is that while we're guaranteed of getting at least a Cat 8 room (and there aren't any really horrible Cat 8 rooms), we could get an upgrade to a Cat 6 or 5 verandah room. On a 14nt cruise, that would be great! So no downsides for us, and well worth the $90 extra it cost us to book the Cat 8 vs Cat 9.