It's funny, many people seem to prefer RCI's ships but I prefer Celebrity's. I don't find anything particularly special about RCI's ships, they seem like scaled down resorts to me, very similar to Hyatt Regency type of ships. Don't get me wrong, they are attractive, but I find Celebrity's ships much, much more stylish. And I find an overall significant quality step up in Celebrity's furnishings vs. RCI.
RCI has a different cruise style as well. While there are some general similarities, and on both lines you get very entertaining bar staff, wait staff style varies a lot, there's more varied entertainment and stress on it on RCI, the waiters sing and dance on RCI and don't do anything close to that on Celebrity, except for an occasional magic trick that waiters will do. There are toga parties on RCI and many more pool games and different types of pool games. RCI cruises have a measure of elegance, you can find ballroom dancing, lectures and the like but Celebrity's a much quieter type of cruise. There's a mingling feel to Celebrity, an early evening feel, and just less planned activities.
I enjoy RCI for a certain fun feel, Celebrity more for tradition and elegance. I think RCI is good for families looking for interactivity. Celebrity is good for couples and families looking for moderate interactivity.