First Class has been around on ships for centuries. The Titanic had it, today it is Grills on Cunard or Retreat on Celebrity etc. Call it what you want, refine it as you want. I'm going to guess that the people who came up with Celebrity Retreat took some market research trips on Cunard Princess and Queens Grill cruises. For example Retreat takes care of the tipping and alcohol which Cunard Grills do not.
Pro of Celebrity Retreat (or Cunard Grills): you can go to the rest of the ship when you want big ship features such as for a large show, a magic carpet (?), more dining and bar choices, nightlife, or the ballroom dancing (Cunard). If you want disco at 1 AM I would not recommend Seabourn, but some people will want to leave the cocoon to drink beer in a pub or dance to Drake at 1 AM.
Pro of Seabourn: you can get most of the luxury food, service, and ambiance at the cost of the cheapest cabin on the ship. We typically do not buy a larger suite on Seabourn (we feel there is no need) & instead stay on the ship longer in a lower priced regular balcony suite. Also important for us is fewer kids with no specific kids features.
They are different experiences. Try them both as see what you prefer. You might like both at different times.