I think, it depends whether you are American or European. We just came back from the Constellation this week. It has really unbeatable Broadway shows, good food and great service. The staterooms w/verandah are small but nice. But is was difficult to have breakfast on the cabin. So, best ship, best service, best shows. I did love it!
But my wife prefers an European ship for several reasons: the food is better with Festival, in the US known as First European. As a suite guest you dine in the speciality dining room. Best food we had.The cabins are larger with tub in the bathroom. There are other reasons as they have five languages aboard an European ship instead of just English; there are a lot of cultures and nations , what is quiet nice to my opinion. People are dressed up better than on most American ships. The temperature in the public rooms on the Constellation was too low to us, on Costa and Festival it´s warmer. We got a cold.
On Costa food is rather bad and service is mostly no good. But for the itinerary (Venice to Istanbul) we will do the Costa Atlantica next fall.
BTW, we are European.