Good choice - she is a nice ship.
I agree with a lot that has been said above.
I am now booked on cruise number 19 .....
P&O cruises are for the party people - age group is more mixed with a balance of the different ages. I like the style of food better on P&O (any time dining which means food is cooked to order).
Princess - age group definately older. My first Princess cruise was a 46 day cruise and the age group was mostly over 50. I thought the length of the cruise was a factorl, but then I went on a 13 day cruise and it was the same. Maybe if you went during school holidays the age group might be younger. I like the set time dining option but not the fact that the food is then cooked in mass and can sometimes be a little like hostel food (overcooked). Still very nice though.
Overall - I dont really care about the ship - I just love cruising and usually pick the cruise I want to do regardless of the ship. We always make our own fun. (we are in our 40s and have 2 kids 10 and 12 who love cruising with us). They love both company's kids clubs for different reasons. Less kids on Princess and more individual attention - more kids on P&O bigger parties, talent shows etc.