Speaking as a Kings Point Deck Cadet from the mid '50s, I feel about three months on multiple types of ships is best. On a passenger ship, you'll need to wear your uniform most of the time, and always be on your best behavior. You won't learn anything about cargo handling and operations, and the second mate on smaller vessels will generally take you under his wing and show you great details of navigation.
Ports are certainly more varied on the non passenger vessels as well. Shore time with the crews of the non-passenger vessels is always more "colorful".