You don't ask much, do you?? 
I've only had experience on the Princess line (2 times; a third in 3 days); and Carnival (once). So this compares the two lines, and take into account my limited experience, and all opinions expressed are mine, alone!
Princess is a sort of middle-to-upper-middle class experience. Where Carnival seemed more a "party" ship, Princess is more laid back, more refined, more subtle. Princess decor is much better. The food is better, and more of it served at meals (I always felt like I should order "seconds" on Carnival, and I'm not all that big an eater!)
Because Princess at least seems more upscale, it also attracts somewhat older clientele -- the cruises I've been on were predominately late 30's and up. Not as many kids. Not as many 20-somethings, partying a lot.
I thought the Carnival ship (The Spirit, a new ship at the time) was glitzy. Lots of bright colors, neon lights, garish decorations (naked people abounded -- and that was the "art" on the walls!) I felt like I was in Las Vegas, one of my least favorite places on earth. Of course, if you like that sort of thing, then the Carnival ships might just fit the bill.
The dining room reminded me of a cafeteria -- a lot of the floor was not carpeted, meaning there was LOTS of noise, not only from the servers, but from the other folks dining. On Princess, the dining room was muted, and made for a much more pleasant experience. The singing and dancing waiters on the Carnival ship were embarrassingly bad. The maitre D was plain awful -- yet he was SO full of himself; I think he thought he deserved his tip for his entertainment! He certainly didn't do anything else to help our table!
of course, that's the good thing about cruising -- there are ships that fit every taste, every budget...you're doing the right thing by asking those sorts of questions, and I'm sure you'll get a variety of answers.
