In ships, tender and stiff tend to refer to rolling periods, with the former having a long rolling period and the latter a short (quick) one.
If you're carrying cargo, tender may be better, the ship rolls more slowly and gently, the movement is easier on the cargo. Tender here means having a low metacentric height (vertical distance between buoyancy and gravity);
Stiff ships tend to snap back upright quickly, with a short rolling period. Lots of military ships are stiff, since the quick rolling period brings the decks (meaning gun decks) back level in a hurry. That 'snappish' motion is hard on cargo and passengers, though, it beath you up.