I don't know your age or interests. If you are a single lady (with the name RayGil, probably not) then you will probably like Cunard better because of the dance hosts.
But for all other reasons as a single I would probably opt for P&O - mostly because it is a more mainstream ship which means you will meet more people you age and preferences.
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.