P and O cruise ship

Since Sydney is a large city, there can be more than one cruise company and ship operating on the same day.
On the day our cruise left, a P and O cruise ship was also in port for the start of a cruise.
From all reports from veteran cruisers we met on board, once you go to a classy cruise line like Royal Caribbean, there's no going back to some of the other cruise companies.
Someone said it was like the difference between a budget and a luxury hotel - like chalk and cheese.
However, some of the other cruise lines may offer cheaper deals of course.