I was on the Celebrity Solstice last November, the Princess Ruby and Princess Crown last month.
The cabins are about the same size, all three ships have flat screens but the new Solstice has interactive TV, including satelite radio and full cabin service for food.
As far as food, the quality in much the same, I was in the new Aqua Class on Celebrity, very nice, but the extra money was not high on my value list.
PS, I am Elite with Celebrity, so many features are iincluded.
Princess has three options that I really enjoy, first and foremost Laundry facilities with an iron set up, anytime dining and the movie under the stars.
The Solstice also has one of the nicest pool decks at sea, it's like a resort.
The coffee is better on Celebrity, except the International cafe on the newer Princess ships, Princess has an extra charge for brewed coffee.