Have not been around for awhile, but figure it is time to stick my two cents into the mix.
We prefer Royal...are Diamond Plus with them....we choose them because it fits our concept of what we want from a cruise.
We also sail Carnival when we have a must get away moment. Do not like the Fincanteri designed ships (going up, then going down or vice versa, when traversing the ship). Food is not quite up to RCI standards....remember, Carnival is the line that throught adding Mac and Cheese to the DInner menu was a step forward last year! But the Steakhouse on Carnival is spectacular (we always eat there 4 or 5 times during a one week cruise...that is how much we dislike the MDR....personal preference, not a food critic).
As for Princess....they fit into the mix as well, not crowded, but same ship designer, and they seem "vanilla" absolutely nothing wrong, but just nothing memorable.
Ok, that is my 2 cents for now.