Well I think we'll all see what happens to Carnival if they continue down the path they're going. I suppose i've just never been satisfied with Carnival. Mind you, I love the experience, but from a technical standpoint they just don't offer what Royal Caribbean does, and I'm not talking about the ships themselves.
Carnival's loyalty program flat out sucks compared to Royal Caribbeans. As a traveler whose held his Diamond Plus membership with RCI for a while now, I can attest to their higher level loyalty program having a completely different structure than Carnivals. While RCI breaks their higher sailing guests off at 24 credits, Carnival requires only 10 and the extent of their program is VIP Pier check-in, a concierge line, some comped laundry and the occasional novelty. Unless something has changed in the past year or so, I just haven't seen the effort.
The second downfall for CCL is their Ocean Players Club. While our family receives discounts already for being TA partners, our cruises are almost always comped by the casino. The 7 Night Oasis trip this may, 7 Night Freedom Trip this summer, 7 Night Independence cruise in the fall and the inaugural revenue sailing of Allure of the Seas are all members cruises for Club Royale members. RCI really knows how to pick up the tab for their guests.
Without question, though, RCI does have their downfalls. Their food is certainly less than fantastic and their crew could often use a couple caffeine tabs to make them smile more. However, when I'm on a ship as the cruise line's guest, its difficult for me to complain.