So I've been on 8 cruises - 7 with P&O and just got back from my 1st Royal Carribean, Rhapsody of the Seas. I was told that once I had been on a R/C, I would never go with P&O again. Also told that P&O = 2 stars and R/C = 5. I don't agree with either of those opinions. In summary, both lines have pros and cons, I will try to detail below. Basically will consider either, but price will be a consideration. Perhaps Carnival next?
P&0 :
Good Points - much better on-ship activities, organised theme nights and more people could buy stuff to participate in themes.
A bit smaller or not as good design as P&O.