Personally, I love Carnival and happen to enjoy myself much more than when i'm on RCCL.
But like people before have said, "I'm sure whichever ship you choose they will have fun!".
Royal Caribbean- Vision of the Seas- Dec. 31, 2006 - January 7, 2007
Carnival Pride- Dec. 18-25, 2005
Celebrity Summit- Summer 2004
And Many more that can't remember off the top of my head!
Cruise Lines I Have Been On
Carnival (many times)
Royal Caribbean (many times)
University of Arizona '12!