The good news is that they are both great! We've sailed on both and the only difference I can think of is that Celebrity seems to have a better handle on the buffet service in the casual dining area--just better organized and wait staff standing by to help anyone who needs or wants assistance in carrying their food to the table. My recollection of Princess is that while the set up looked prettier, it was actually pretty disorganized. I took my Mom along on a Celebrity cruise this past spring and she loved it. The wait staff were extremely charming and courteous to her. So rather than picking a cruise based on the ship (or line), I'd focus on the itineraries to help me decide.