We've had great luck on both RCCL and Celebrity. Our now 7 year old son has been on 3 RCCL cruises (all 1 week long) and 1 Celebrity cruise (2 weeks long) and loved them all. He's actually worried if he'll like Disney as much!
There were fewer kids on the Celebrity cruise than there were on the various RCCL cruises we've done, although there were still a fair number of kids on board. This did not bother our son at all; he liked the personal attention in the kids' club and on the ship. He liked some of the things for all ages (not just kids) on the Celebrity ship (the Eclipse, in our case) that the other ships just don't offer. He loved the glass-blowers on the top deck, the large grass lawn, and the creperie. He really loved our Celebrity cruise.
RCCL has great kid's programs and great swimming pools. They are not as good for under 3 and not potty-trained as I understand Disney is, but they've improved that over the years somewhat. At this point, most of their ships have a toddler program, but no nursery for dropoff, and no pool area for the unpotty-trained. They do have a toy check-out program, where you can check out bags of toys at no charge for little ones. They do have a daily playgroup (parents must stay) for babies and one for older toddlers. On our February cruise, they set up one room as a basically permanent indoor playground for the toddler set, that you could take your young child to at any time from something like 7am to 10pm, except during the hours they were doing the playgroups in the space. For older kids, the Adventure Ocean clubs had activities going through most of the day except mealtimes. I will admit that we got a little frustrated that the clubs often seemed to be closed when my son would want to go while we did a longer sit down meal. Anyway, we were generally very impressed with the quality of the club and the staff on RCCL.
