We did the 4 day land/3 day sea this past April - and wish we had done it in the reverse - we would have loved to have the additional day on the cruise. (It was our first time cruising, and we had chosen to do the 3 day just in case we didn't like it . ) I would say it depends on the ages of your children - ours are teens, and knew exactly what they wanted to do and what could be skipped at the parks. By going to the parks early, and being organized, we were able to do everything they wanted to do, but to be honest, we were exhausted by the time the cruise came.
Then, on the cruise, we were up and off the ship fairly early in Nassau for the excursion to snorkling with the stingrays - we didn't get back to the ship until late afternoon. The following day was at CC, which is so gorgeous you never want to leave. Then it was back to the ship to have dinner, pack and try to enjoy what's left of the evening. We really would have liked a day at sea to enjoy the ship - 3 days was just too rushed. If I ever do 3 day again, i will stay on the ship in Nassau - we were just not impresses with Nassau. Next trip will hopefully be a 7 day - it is just one awesome trip!