Cruises take care of most of planning. You don’t have to book each leg yourself as you usually do on a train.
The cruise line offers a calendar of activities and entertainment on board every day, so that you can choose as you wish.
However, there is no flexibility with regard to time spent stopping the port. Cruise ships comply with a tight schedule, and if you are late, you can be left behind unless you are in excursion that you have reserved via the cruise line. But if you plan your own train trip, you can give yourself as much time as you want.
During my cruises, most port stops lasted about eight hours. I reserved excursions through the cruise line, including beach days, hikes and visits. Some were convincing, but in the end, I did not feel that I had enough time to have more than a taste of destinations where we stopped.