It seems to be about an even mix between tuxes and dark suits most of the time, perhaps occasionally swinging as far as 70%-30% one way or the other. I have taken seven cruises and have yet to see a situation in which a gentleman wearing either a dark suit or a tuxedo would feel inappropriately dressed.
As I noted in an earlier thread, the gentlemen who wear dark suits tend to be gentlemen who own a dark suit because they wear it on other occasions, either professionally or socially, and do not own a tuxedo. There are three groups who tend to wear tuxedos -- those who own tuxedos (usually because they wear tuxedos on other occasions), those who don't own dark suits (it's much cheaper to rent the tux than to buy a suit), and those who own a dark suit but rent a tux anyway because they (or their ladies) think it's classier to wear a tux on formal night.
The bottom line is that each gentleman should wear whichever is more convenient on formal nights unless he has a strong personal preference one way or the other.