I have cruised aboard MV Sun Princess (twice), MV Sea Princess, MV Grand Princess, and MV Ocean Princess. There are five noticeable differences between MV Sun Princess and her newer two sisters.
>> 1. At the aft end of the Baja Deck, MV Sun Princess has full suites inboard and minisuites outboard. All three newer sisters (MV Dawn Princess, MV Sea Princess, and MV Ocean Princess) have full suites outboard and minisuites inboard.
>> 2. The bridge wings are open aboard MV Sun Princess (and, if I remember correctly, aboard MV Dawn Princess). MV Sea Princess and MV Ocean Princess have enclosed bridge wings, with the observation area above the bridge extended over the bridge wings.
>> 3. The treatment of the area between the two main swimming pools is somewhat different. MV Sun Princess has a waterfall on the front of the upper pool and a footbridge over the stream from the waterfall to the lower pool. MV Sea Princess and MV Ocean Princess have spray jets in the front of the upper pool and no footbridge.
>> 4. Several of the public spaces have different names (see the diagrams in the cruise brochures) and the arrangement of furniture varies in the Wheelhouse Bar to accommodate display cases of different sizes.
>> 5. The original artwork is, of course, completely different.
If you have taken a cruise on any of the four sisters, you will feel right at home the moment you set foot on any of the others.