I find that there are people who love one cruise line only and will look at another one in a different view. I have sailed both cruise lines and had a great time on both of them. I can tell you my own personal reasons why I prefer RCCL, not to say I wont go on carnival again, but these things tend to make me lean towards them when picking a cruise.
First after 5 carnival cruise, no discounts were ever offered, only those member cruises. Rccl upgraded me early and after the freedom I will be a diamond member, which means $200 off a balcony whenever I book (right now as a platinum I get $100), plus if you book a sailing with a coupon you can save almost $400, that is one of the main reasons I end up with them because I save money.
Smaller reasons are, I love that RCCL leaves their pools and hot tubs open all the time, carnival closes them so early, I cant tell you how many nights we have gone up on deck and enjoyed them. I also love the adult only section of the ship, especially the pools, nothing against kids, but sometimes I just dont feel like being splashed all the time. I also like the way the pools are set up, you have to climb down into them on carnival, I love that ledge all the way around the pools to sit on, its great.
None of them would stop me from sailing carnival though, I loved every cruise I had with them and enjoyed all of them, but the money I save with RCCL always seems to lead me back lately (and I love those voyager class ships, cant wait to see the freedom)