Serena & Concordia are clones of Carnival Liberty/Freedom ships, different wallpaper & paint. Any comparison with RCI & Princess 4 star lines & Celebrity 5 star line is surely a non starter. Costa are a 3 star line & surely should only be compared with similar grades, MSC, Island, Thomson etc.
Costa cruises cannot even begin to compare with Princess etc. If you have been on Princess or Celebrity don't even think about going on Costa-the food,service,entertainment etc is rubbish!Its like the difference between a Gordon Ramsay restaurant and going to McDonalds!