I booked a 5D. It's on deck 6. I'm pretty sure that 5Cs are on decks 7 and 8 (and 5Es are on deck 5). Other than that, the layout of the rooms are similar.
This is our first cruise, but from what I've read the higher decks are "more desirable" due to their proximity to the top deck amenities. However, from what I read, if you might be prone to motion sickness, lower decks are better because they don't sway as much.