Being a long time Carnival cruiser, we went on a Princess cruise last year.
I think both lines are great. Princess might just be a little better in the food catagory, but Carnival beats everyone in the night life catagory. I really liked Princess having fresh water pools. I know Carnival gets bashed for being a "Party Cruise Line". I can't understand why people knowing this choose to cruise with them if they don't want any kids on board, or a more fun loving type of cruiser. Go on Holland America if you want quiet days and nights that end at 10pm. That's why there are so many cruise lines to go on. There is something for everyone.
Having added my two cents, I will cruise with Carnival or Princess anytime. We have a great time on any cruise we go on!