I think I will rent one next time I cruise. I've had friends who have done so and really like not having to drag a suit on board with them. The tux, shoes and everything are hanging in their closet when they arrive on the ship the first day...kinda nice. I think you can also rent them once you are on board if you haven't done so before your cruise.
Also, tux looks great for your formal night pics. Suit is nice but nothing looks quite as good for guys as a tux in my opinion.