Princess is my favorite line, because it feels like home to me. They're not stuffy at all, but just a little more elegant than Carnival. I've enjoyed every Carnival cruise I've taken (getting closer to 10), as well as NCL, HAL, Royal, and Costa. Celebrity was not my favorite, but still had a great time, just not my line of choice.
I, personally, prefer the southern because the water is a little warmer and the snorkeling (depending on the ports) is a little better. Belize and Roatan are wonderful snorkels, too. Dominica and Grenada are my favorite snorkel places.
You've asked a very hard question, because what I like you might not. This is totally my own individual opinions. Hope it helps a little.