So far we have been on 3 carnival and 2 Princess cruises....my comparison
Food....Carnival wins hands down, from the main dining room to the buffet and specialty restaurants Carnival is much better.
Cabins....Princess's cabins are a little bigger I believe and they are nicer IMHO.
Entertainment....Carnival wins hands down, Princess not even close. Last Princess cruise most lounges were closed by 11pm and the shows, while good wern't as good as Carnival.
Service.....Carnival wins, no comparison....Carnival's waiters and room stewards and bar tenders do a superior job, much better then Princess.
Movies under the stars.....Princess excels here with padded lounge chairs, blankets if chilly out and better movies.
Overall Value.....Carnival, hey why pay more for a lesser product?
again, these are my opinions, my D/W and I are in our upper 40's and like things a little livelier then the Princess crowd seemed to. Last cruise on Princess we were probably some of the youngest on the ship.
Hope this helps