I forgot to mention it also depends on whether you want your wedding on embarkation day (the day the ship is set to sail) or at one of the ports during the cruise. If you choose one of the ports, I've heard of brides not going through the cruise line and using the resort planner or other destination wedding planner. I think Royal Caribbean and maybe another cruise line give the option to get married at sea (on a sea day), but it's more expensive than the other packages.
As for the packages Carnival offers, I think it depends on what you want. Most of them (if not all) include up to 20 guests in their packages so you would not have to pay for extra people. I have heard that the luncheon is more of a sit down meal with the family and the deluxe package has more of the dancing and music with guests being able to eat your choice of different hot and cold hors d'oeuvres during the reception.
Ok now I'm done! I hope I answered at least one of your questions!