We are Elite with Princess and started sailing with them when they were NOT part of Carnival. Experience totally changed after that and not for the better. First sailing on X was on the old Horizon when Michel Roux was the Executive Chef. Food and service then were top shelf. We are also Elite+ on X but until Luminae came along, they couldn't hold a candle to O's food. In 2014 we took our first O cruise and never looked back. Amazing food( blows away Luminae), wonderful cabins, and a nice relaxed atmosphere. Also, O is more port intensive so if your goal is to actually see the world and not just your cabin, O is the choice.
A caveat, we prefer the O class ships, Marina and Riviera, over the former Renaissance Cruises ships ( the R class). More room, more dining options, roughly the same size as the Millennium Class on X but with half the passengers. Give it a try. Only you can determine if it's for you but you'll never know if don't try it.