In our opinion, Carnival ships are more of a "party" atmosphere with generally a younger crowd. Often very affordable trips as you can see on Expedia or whatever.
Royal Caribbean is more of a mid-line cruise with a mix of passengers. We felt it was not as "flashy / party-like" as Carnival.
Our favorites was Princess Cruise ship. Definitely had an older crowd when we cruised. We felt the caliber of the food was better than any other ship. Our kids LOVE cheesecake and the other lines it was "fake" cheesecake - not New York Style like they love. lol
Another expense not always considered by first-time cruisers is the COST of shore expeditions. If you plan to see the island or Mexico or whatever.... be prepared to pay. Generally, excursions booked on the ship are $75 - $125 per person, per excursion. If you go to Cozumel and are happy walking around the town you can do that cheaply and enjoy some good food too. Any of the "really cool" sightseeing trips are usually quite costly - just thought you'd want to know.
I don't know if I can post the name of another site that has great boards on every cruise ship, every itinerary, every port of call, etc ??? I'll give it a shot and hope I dont get in cyber-trouble LOL
I love cruisecritic . com Great site to get info, though I saw you had a couple of folks give you some good stuff here too. Flyertalk ROCKS.
Happy cruising - maybe we'll see you in February on the seas!