As you can see different cruise lines are like different hotel chains. Many of those lines are owned by one company, Carnival.
One thing that is important are the shore excursions. Do your research long before you get anywhere near a ship. The ships concierge will hard sell whatever they have an association with, mainly because typically a ship gets at least 35% commission on the excursion. Do your own research & you will find excursions not offered by the ship. Often you can find better excursions same money or ship offered trips for less money.