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I think Princess has a "Coral"-class ship ("Coral Princess" or "Island Princess") operating that itinerary, right? If so, the ships (Carnival uses "Carnival Spirit") are fairly similar, the main difference being that Carnival is more Las Vegas like while Princess has a more European style. As for the product, Princess is considered somewhat more "upscale" - even though both cruise lines are in the standard segment of the market - while Carnival offers a somewhat "younger" product.
Personally, I would consider either a good choice. Sorry, I guess that doesn't make it much easier for you.