We just returned today from a 10 day Alaska cruise out of San Francisco. We chose Princess Cruises simply for the fact that we live about 40 minutes from the dock so we didn't have to fly anywhere. That was huge! Since my wife had no airline restrictions to deal with, she packed 7 pieces of luggage to be checked. Fortunately, she learned her own lesson there so I didn't have to add my 2 cents. That's dangerous when she does all the packing anyway.
Check in was fantastic. From being dropped off on the street to our stateroom was about half an hour. These guys were organized. We had a balcony room, our first. Nothing fancy but quite adequate and thanks to the space under the bed for suitcases, we didn't have to discuss the 7 suitcases any more.
I thought Mariner of the Seas was a nicer ship - flashier. Sea Princess is older and it's easy to see that. Definite wear & tear but certainly not anything that would deter us from returning. Service was great and embarkation was smooth too.
I think it was the food in the buffet that was my problem. The dining room was great but the buffet was horrible. On the Mariner we ate most of our meals in the buffet but on the Sea Princess, we made a point of getting to the dining room on time to avoid an encounter with the buffet. It's too small, most of the food was like a terrible high school cafeteria and the service was slow. This was our only complaint on the ship.