If you're going to be "cabin moles" who rarely show their faces in public while aboard ship, opt for MV Grand Princess or MV Golden Princess. Their cabins are slightly larger than the cabins on MV Sun Princess and, in general, have a superior layout. If you're high rollers who like a fancy suite, MV Grand Princess and MV Golden Princess also offer more alternatives as to the type and size of suite that you might select.
I'm not exactly a "cabin mole." I go cruising to meet people, to participate in some of the activities offered aboard ship, and to relax and enjoy the sea-going life. Given the choice that you postulate, I'll cruise aboard MV Sun Princess, MV Dawn Princess, MV Sea Princess, or MV Ocean Princess any time.
>> The public spaces aboard MV Sun Princess, MV Dawn Princess, MV Sea Princess, and MV Ocean Princess are much more functional than the public spaces aboard MV Grand Princess, many of which are simply too small for the capacity of the vessel.
>> MV Sun Princess, MV Dawn Princess, MV Sea Princess, and MV Ocean Princess invariably offer enough concurrent activities that I'm forever having to skip something in I really would like to participate. The additional activities aboard MV Grand Princess only make the choices more difficult and the add to the regrets that I could not do everything that I wanted.
>> The public spaces aboard MV Sun Princess, MV Dawn Princess, MV Sea Princess, and MV Ocean Princess are large, open, and spacious, so finding a place to sit (or whatever) for an activity is never a problem and none of the spaces feel even remotely closed in. Several public spaces aboard MV Grand Princess (and probably also aboard MV Golden Princess, though there was an attempt to correct at least some of the problems on the latter) -- some of which acrually are much smaller than the equivalent spaces aboard MV Sun Princess and her sisters -- are simply too small in relation to the capacity of the vessel for the activities held there. For example, I often could not find a place in the Wheelhouse Bar, which has live music for dancing, after the show and there were so many people who came to play bridge that they could not fit into the card room when I was aboard MV Grand Princess. Additinally, several of the public spaces aboard MV Grand Princess, including the atrium and the Vista Show Lounge (due to a low overhead), feel very claustrophobic. The atrium aboard MV Golden Princess has a modified design that might not be as bad, but I doubt that the designers can do anything about the low overhead in the Vista Show Lounge.
>> MV Sun Princess, MV Dawn Princess, MV Sea Princess, and MV Ocean Princess have a full wrap-around promenade route on a single level for those who wish to walk laps. Aboard MV Grand Princess and MV Golden Princess, you have to ascend the equivalent of three normal flights of stairs, go around the "bow walk" that appears as a dark stripe in the brochure photos of these vessels (called the "upper promenade deck" in the deck plans), and descend the equivalent of three normal flights of stairs to return to the main level of the promenade.
>> There are several other public rooms aboard MV Grand Princess with very poor layouts. The Horizon Court is split into two separate sides that don't interconnect, so people who arrange to meet there won't find each other if they enter on the opposite sides. The Botticelli Dining Room (MV Grand Princess) or Canaletto Dining Room (MV Golden Princess) are in extremely awkward locations with no "gathering space" (lounge, etc.) near their entrances, so the throng waiting to enter backs up onto the stairway. Additionally, these dining rooms get obnoxious noises and vibrations from the stage elevators for the Vista Show Lounge located directly above whenever they raise and lower the stage. MV Sun Princess, MV Dawn Princess, MV Sea Princess, and MV Ocean Princess don't suffer from this sort of problem.
>> Finally, MV Sun Princess, MV Dawn Princess, MV Sea Princess, and MV Golden Princess have magical teak deck lounges on the Promenade Deck that are absolutely the most wonderful place to relax on an afternoon at sea. :-) You have to try one to understand its magical properties, but be sure to do so on the SHADY side of the vessel! Alas, the promenades aboard MV Grand Princess and MV Golden Princess are not wide enough to accommodate these wonders!
Given a choice, I'll take MV Sun Princess, MV Dawn Princess, MV Sea Princess, or MV Ocean Princess any time over MV Grand Princess or MV Golden Princess. It's no contest. Then again, I'm not a "cabin mole" -- so look for me out and about the public spaces aboard these vessels.
Have a delightful cruise, whichever vessel you choose!