the last time i went on carnival was 2001, carnival destiny
but i HAVE been on two NCL ones, Norwegian Gem a week ago, and Norwegian Dawn last summer.
Norwegian isn't strict at all really in my opinion. no curfew, one of the nights i got way too drunk, ended up wandering in the teen club and didn't really end up with any problems with security lol. i guess this was mainly because there was only one teen director, and she was cool. but yeah, the gem only had ONE teen director there, and the teen program was really weak. if you are 18+, there's still nightclubs and drinking and plenty of partying, it's not all old people. the service in the restaurants is nice, but nothing crazy upscale, it's casual dress if you want all week. alcohol was easy to 'smuggle' on board. there was lots of activities like karaoke, deck stuff & the rest of the normal things. the shows were also pretty cool, broadway style.
norwegian is pretty fun, i guess it really depends on if you go at a popular time for kids to go (summer, spring break/winter week)
but princess > ncl in so many ways =)