We did Grand Princess last year. We haven't been on Pride or Celebrity but will be going on Carnival Legend out of Fort Lauderdale in 2 months and can let you know what we think.
I wasn't aware that Princess wouldn't be operating out of FL next year - is that that just for the summer period?
We have cruised out of both Miami and Ft Lauderdale. The Bayside Marketplace in Miami is great for activities on the day prior to the cruise, but I find cruising from FL easier due to proximity to the airport and most hotels will include the transfers. I realize that your preference is to sail from FL, however if there was a better deal out of Miami, I'd be happy to leave from there.
For a group of 4 in a cabin, in our experience we have found that the average cabin on Carnival provides the most space (compared to Princess, RC, NCL, Cunard). Cabin sizes were all sufficient however, and they were close enough in size that we wouldn't use that criteria as a priority for deciding on which cruise to take compared to other factors. The exception to that rule would be our cruise on NCL Norwegian Sea which was really small.
We did a Christmas cruise on Destiny in Dec 98 with our children as part of a family group and found that the number of children on board was so large that children often had to wait around quite a bit to participate in popular activites. Although there were lots of activities to do, our children and nephews joined us at the pool frequently. As there were four of them - our two children and two nephews, they were able to play together and were less reliant on the ship's programs for activities.
I'm not sure our recent cruises would be a good indicator of 'family experience' as you are travelling in the summer and we usually avoid summer and spring break so when we cruise there are fewer younger travellers on board at that time. We took our children on the Grand last September and there weren't a lot of kids on board, but there were enough for group activities.
Our experience has been that group activities/supervision will always be subject to the enthusiasm of both the staff and partcipants (whether adult or children) so we wouldn't really give one cruise line a higher recommendation than another, although Carnival generally has more activities available to choose from. For a second opinion, and to put it into perspective, our children (now young teens) preferred the Carnival program.
Quality and service are very subjective areas and I realize many people have very specific ideas based on their cruises. I won't even go into the food/food service debates - we go on board with a glass half-full approach expecting to enjoy ourselves and leave it at that. I find that there are a number of factors that influence your cruise that have little to no bearing on the Cruise Line, including friends you meet at dinner assignments, weather during the cruise, if you are travelling by yourselves or with a group/family, what you choose to do at ports of call(perhaps exclusive of the ship tours) and how those excursions turn out. Suffice it to say that we'd go back to Carnival, Princess or RCL in an instant with a slight preference to Carnival.
Although all of this has not necessarily been directly on topic, I hope this may be of some help.
I've been on 14 cruises. Three of them on Carnival and two on Celebrity. On Celebrity I went on the Infinity and the other on the Galaxy. If you have to take a cruise, go on any of the Celebrity Cruises ships, and that incluides Galaxy. The service is 200% great and you wont get disappointed.