You really can't go wrong with either. If you liked CCL you will most likely enjoy RCCL. I have tried both and liked both. Pick the cruise for the ship, and the itinerary and budget that suits you and you'll be happy.
But go ahead and try different lines, who knows, maybe you'll like Princess best... they are all slightly different because they try to appeal to varying tastes.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007