Have been on 5 different CCL ships (and leaving for a 6th in 17 hrs, 36mins), and the first formal night seems to be a little more formal than the second. More women in sequins, ballgown type dresses and a few men (generally mid 20's to mid 30's) in tuxes. The second night is more cocktail dresses and dark suits. I pack according to that and take one rather formal dress and a cocktail dress.
After LOTS of cruises, I have never worn anything even remotely resembling a ball gown. Lots of women do. My husband (who's only in his 30's regarding his waistline) always wears a black dinner suit for the first night, if we're in a warm climate he will wear a white dinner jacket the second night.