It depends what you are looking for.  Are you sailing primarily for itinerary or are you primarily sailing for a ship with the most amenities for a 3 year old?  When my oldest was 3, we sailed primarily for itinerary so we were on a bunch of incredibly old and unimpressive ships that lacked all the bells and whistles of newer lines.  Princess has no waterslides, zip lines or bumper cars, but they have an excellent kids program.  And because my oldest had never experienced those things on a cruis