We have been on both Marella (Spirit,Celebration, Majesty and Discovery) and P&O , (Ventura , Azura, Britannia, Iona and Arvia) . In my opinion there is not a lot of difference between the two apart from the newer P&O ships. I found the Entertainment as good on Marella as it is on P&O. I also found that some over indulged in the all inclusive on Marella and became rowdy and drunk even in the shows on Spirit.
That said we also had a recent experience of that on Britannia but only an isolated incident. I have left a full review in the reviews section and have a look at it if you want and it is a fair and Honest report of our experience. I guess if its a 10th anniversary sailing you are going on they will try to make the trip more special.