I have cruised both. IMO, and to echo what has already been said, Princess was more quiet and "relaxing". Carnival was more of a party atmosphere. For example, on the pool deck on Princess there was no blaring music or pool games. Lots of folks chatting, reading or snoozing. On Carnival, there was loud music and at times pool/deck games. Another example, on Princess no singing by the staff in the dining room. There are little dining room performances on Carnival.