First difference - the price!! I've done RC quite a bit but the prices started to increase above my budget. As the on board currency is USD, and pound is very weak (Thanks Brexit! ) the drinks prices are extremely high.. plus them being an American ship you also get the added 18% gratuity. P+O onboard currency is the pound and drinks are not overly expensive. You can also take 1lt on per person. So if you are a drinker, then P+O is certainly better value.
Secondly the type of cruise. RCL is all singing all dancing where as P&O is a little more subdued and a bit more formal. They still have formal nights where black tie is expected.
Iona is a beautiful ship and P&O has really grown on me. It does not have all the 'wow' of RCL Oasis class but more in line with Princess.
Food - I have had bad food on RCL and P&O but personally I prefer P&O.
I've sailed RCL, NCL, Carnival, Celebrity, Princess, MSC, Fred, Cunard and P&O and they all have their unique plus points, but P&O are good value for money. Celebrity and RCL are my favourites (if money was no object) and NCL/Cunard are at the bottom of my list.
Thing is, we all have different views !