Here's my two cents:
I am in my early 40's, and cruise a few times a year with others I work with (group anywhere from 8 to 16 people). I have sailed 4 times on Carnival (most recently Dec 2003 on the Pride) and Princess for the first time this past May on the Golden. I am scheudled for my next Princess cruise this December aboard the Sun.)
Here's my comparision:
Carnival: Hit and miss. Ranges from good to awful. Fish and seafood dishes are over-cooked, as if spending long periods of time under a heating lamp in the kitchen. On our last cruise, many in our group had to send their lobsters back, as they were dry and like chewing on a rubberband. Lamb dishes were very good, steak was not. Some dishes were not hot, and just barely warm. Desserts mostly made of gelatin, look very attractive, but taste is mediocre. Buffet food on Lido deck not good at all. Room service food is very limited in variety.
Princess: Never had a bad meal. Fish and seafood dishes were juicy, hot and cooked perfectly. Steaks were cooked to order and hot. Desserts were out of this world. Food in the buffet Horizon Court had excellent variety and high quality for a buffet. You can order any dinner item to your room if you like through room service.
Carnival: Still traditional dining times. Wait staff seemed rushed on our last Carnival cruise. Waiters perform in tacky shows each night, the iterupts the flow of the various courses. Had long waits for water and bread several times, while "entertainment" was going on. Some at our table would still be on their appetizer course, while others were already being served their entree. Uneven service.
Princess: Choice between traditional dining times or "personal choice" that allows you to eat where and when you like. Once had a ten minute wait for dinner, on all other nights we were seated immediately. No dining room entertainment to interrupt the flow of service and courses.
Carnival: Standard balcony room on Pride was very large and spacious. Good sized balcony and plenty of closed and drawer space. Nice table top alongside wall that we used to set up a bar. Good size bathroom and shower.
Princess: Nicely appointed, yet smaller cabin size than Carnival. A less closet space and fewer drawers. Bathroom and shower much smaller than Carnival. On the Golden, the lower balcony cabins had huge over-sized balconies, but were only semi-private.
Carnival: Varied. Regular dining staff and cabin stewards were friendly. However, those that were not assigned to you were indifferent at times. (Example: walking down the hall passed someone else's cabin steward, you will not be greeted or acknowledged.)
Bar staff varied. Some friendly, others not.
Princess: Absoultely felt service was much friendlier and personal. Greeted by all staff and crew when you walked by. Bar staff was attentive.
Carnival: Focus on Vegas type productions (I live in Vegas, so this is old hat to me.) Good vocals and choreography. Varied entertainment, with several shows per night.
Princess: I think this is there weak area (though I don't cruise for the entertainment, and don't mind missing shows)/ Performers were sub-standard. many vocals are lip-synced, which is annoying. The main entertainment on two of the nights was simply putting a movie screen in the theater and showing a nine-month old movie.
Carnival: More families and younger folks, though there are many passengers who were seniors. Some teens up past the ship's curfew running down the halls and playing on elevators.
Princess: Less families, kids and teens. Good mix of ages (20's to ??). No one group dominates, though a little more in the 50's age group.
If you are looking to avoid kids and teens on any cruise line, you should do as our groups does, and book at appropriate times. Avoid summers, Thankgsgiving week, Christmas/New years holdiays, Spring Break time, which runs from early March through mid-April, and Easter week. Cruises of 10-days or more also attract less families. We usually book in early December and early May, and find very few families during this time. I also hear that September and October are good kid-less months, as parents are usually less likely to pull their kids from school when the term has just begun.
Overall, Princess is for me. I enjoyed Carnival when in my 20's, but have out-grown the line, and want a better cruise. I cruise for the food and service, and Princess has these two items down over Carnival. Carnival has better cabins and entertainment, and a more party atmosphere if these things are important to you.
Princess is not a "formal" cruise, but more of a nice casual-elegant "country club" type atmosphere. Very relaxing.