It's no contest. I like MV Sea Princess (and her sisters) much better than MV Grand Princess, and those four or five consecutive days at sea would be absolutely wonderful!
By way of amplification, my idea of the Utopian ship would be the basic deck plan of MV Sea Princess with the cabins of MV Grand Princess. Although she's a beautiful ship in many ways, MV Grand Princess has several boneheaded design flaws that are very annoying to me as a passenger. I really like the positioning of the closets in the cabins of MV Grand Princess to form a hallway leading to the head, such that the light in the head does not shine directly into the room (read: the eyes of a sleeping roommate) when the door to the head is open at night. Nonetheless, this feature is far outweighed by bad design in areas such as the following:
>> Several of the public spaces aboard MV Grand Princess seem almost claustrophobic, incuding both the atriusm and the Vista Show Lounge, due to squatty overheads.
>> Some public spaces aboard MV Grand Princess, such as the Wheelhouse Lounge and the card and game room, are too small and confinng for the number of passengers aboard the vessel. At bridge gatherings, for example, the organizers had to set up additional tables in the atrium -- where they did obstruct passage to some degree -- to accommodate all of passengers who wanted to play.
>> The public spaces aboard MV Grand Princess seem too disjointed, such that there's really noplace that can satisfactorily accommodate "all passenger" events such as the Captain's Cocktail Party (usually held in the atrium aboard MV Sea Princess) and "Island Night" (held in the main pool area aboard MV Sea Princess).
>> If you are walking laps on the Promenade Deck of MV Grand Princess, you have to ascend and descend about three flights of stairs near the bow to connect from one side to the other. (There is a jogging track around the lap pool in the upper level of the fitness center, but you will do many more laps per mile.)
>> The Botticelli Dining Room aboard MV Grand Princess is in a very awkward location.
>> The promenades on the Pomenade Deck of MV Grand Princess are too narrow, and thus don't have the magical teak lounges that you will find aboard MV Sea Princess. (Caution: Always use the lounges on the shaded side of the Promenade Deck, as they may make you completely oblivious to the passage of time and thus very susceptible to severe sunburn.)
>> The Horizon Court aboard MV Grand Princess is divided into two separate areas that interconnect only via the outdoor seating area by the aftmost swimming pool. You can plan to meet somebody on the Horizon Court and not find each other if you enter on opposite sides.
>> The pizzeria at the top of the atrium aboard MV Sea Princess is a personal favorite, as both the pizza and the atmosphere are extraordinary. Neptune's Pizzeria aboard MV Grand Princess is out on deck, its offerings more limited (two pizzas at a time, served by the slice), and any semblance of "atmosphere" is totally lacking.
As to choices, MV Grand Princess may have more choices of activities -- but when you can't do everything that you want to do aboard MV Sea Princess, what difference does "more" make?
Would I refuse to cruise again aboard MV Grand Princess? No, but I certainly would choose MV Sea Princess (or any of her sisters) over MV Grand Princess any time, all things being equal!