My brother has sailed on 2 Carnival cruises, and then joined us on his first Royal Caribbean cruise.
His feedback - no comparison.
His take was that Carnival ships were "older feeling onboard" for cabins and other locations, the maintenance was clearly overdue on railings and decks, and that the Royal Caribbean ships were "like new in comparison".
Then there's the past year of Carnival debacles...
We'll stick with Royal Caribbean.