the only reason I cruise both Carnival and Royal Carribbean is due to the fact that I am only hours away from several ports and I pick the best date, price & iteniary that I can fit into my schedule. Each line (although many will disagree w/me) are almost interchangeable; the slight differences are:
1. Carnival rooms are bigger, but RC have more appealing decor.
2. Carnival shows are better, but RC has more appealing decor....
3. Overall Ship: carnival a little glitzy and RC has more appealing decor..
3. Fellow travelers; Carnival a little more of a party; RC - more appealing decor,
Seriously RC tends to be a little more "stuffier" but nothing that would keep me from having fun or sailing on a RC cruise - I jsut won't sail in height of the senior citizin sailing time); and on Carnival never saw a party get out of hand that would keep me from sailing with them again (but I try to avoid the spring break sailings --sure I have see a drunk or two asked to chill out, but that happens when alcohol is involved and it happens in a lot of places, not just on cruise ships)
dining room food great on both lines, felt like Carnival had more choices; buffet food: it all tastes like bland bufett food on both lines, but RC has it laid out better; with stations, etc.. (again, more appealing decor).
5. Casinos: always had better luck on Carnival, but RC has more appealing decor...
So I say go for it, try out a RC cruise and compare for yourself. There is not enough difference to make you "hate" it..