My Travel agents are high on the "dam" ships as well. As far comparing Holland America to Carnival, that is all accoring to your taste. Carival is the "fun ships" which are more casual, hip, and energetic. Holland America is a classic, refined, and relatively more dignified experience. Some might say Carnival is all party drunks and HAL is nothing but old farts. At least from my trips, neither stereotype is true.
Whoever this so called travel agent is, they cannot be very creditable. People have different taste and therefore no cruiseline can be "better" than another.