I've been on several Princess, 1 NCL and 1 Carnival cruise. After the Carnival, I decided to stick w/Princess. I'd do another NCL if the right opportunity came up.
Princess is definitely the best at crowd control, embarkation/disembarkation. Carnival was severly lacking.
I love having freshwater pools/spas open 24/7. Nice for a late night swim.
I prefer the layout of the buffet on Princess (several stations, so you don't have to go through the whole line to get one thing). However, Carnival had a nice deli w/made to order sandwiches, very tasty.
I thought the Carnival Glory had the ugliest decor I'd ever seen. It was dark & gloomy, where Princess is much more elegant with their pastels, brass/chrome & glass.
On Princess, the bars/show lounges are spread out over the ship. On Carnival Glory, they are all grouped at one end. You couldn't stand outside of one without hearing the noise of the other, and they were so crowded, we couldn't get inside.
One thing on Carnival really bothered me. A gentleman collapsed while we were waiting to board the ship and it took several minutes to get a medic up there. Personally, I found that totally unacceptable! Fortunately, he was ok, but had it been serious...well, I would have been quite upset if it was my family member. If they are going to pile you in like cattle, they should have help stationed in several locations.
Overall, we much preferred Princess. Not that I wouldn't do another Carnival cruise, but if I did, it would have to be an exceptional deal to get my interest.