Hey Presto!
We have sailed on both and they are similar in some things but not in others.
Advantages of Marella for us, is adult only and inclusive drinks packages available.
P&O are stricter on formal nights, my Wife forgot to pack my Tuxedo one Christmas and we were not allowed into certain bars and the MDR on formal evenings, Marella are not so strict, we can take or leave the formal evenings (It's hot in the Caribbean wearing a Tux) but we also enjoy them, more so on Celebrity.
We like the Limelight club on P&O, it's intimate and entertaining.
We think the entertainment on Marella is better than P&O.
We preferred the room on Marella but that is being picky.
We chose Marella this year mainly because the itinery has 2 new islands we haven't visited before, the inclusive drinks is a big thing for us, we don't go mad but it's nice to know you can drink anything anytime.
Our decision is usually based on itinery, both cruise lines are fantastic, our preferred cruise line is Celebrity but we were happily surprised at the quality of Marella and we think it's as good as P&O.