I am sailing overseas on QM2 and home on the Grand Princess. Should wear the same dresses for the formal nights on each ship. I have had some really fancy dresses made for QM2 because we are sailing in queens grill, but I don't want to be over dressed on Princess. I know that the Grand is also a very nice ship, but I thought that Cunard made more of a big deal of fancy wear than most cruise lines. I have almost a week in Europe where I will have to schlep all my luggage around, so I am trying to be conservative.
If you are talking about fancy ballgowns then you would be in the minority on Princess ,if however you are talking about very nice long dresses, then that's what the majority of the women will be wearing. Some ladies opts for long dressy skirts with dressy or sparkly tops, some opt for dressy cocktail dresses and some wear dressy pant suits.
I would pick the dress or two that you love, ones that aren't ultra fancy and you will be just fine.