Itinerary first but the ship, with having been to most of the stops in the Caribbean, are fast becoming as important as itinerary.
What do I look for on a ship?
size first! Small ships give my wife sea sickness because they aren't as steady in the water.
How old is the ship? Does it have balconies?(some of the older Carnival ships have very few)
These are really the only things that concern me about the ship as most cruise lines are very similar as far as entertainment and food goes.