Without being a party pooper, when are you planning on going ?
We booked Britannia for a Caribbean cruise in December 2023 a couple of months ago, and the cabin choice was already pretty limited.
I assume you are looking at 2024 - perhaps if you check availability now for the type of cabin you want, it will help you decide.
As an aside, as previously mentioned, whenever I have priced up a Marella cruise versus P&O (select type fare, precise balcony location etc) Marella have always been considerably more expensive than P&O, for no obvious benefits, apart from a few different ports, which imho, is not that important in the Caribbean.