I have already spoken my piece about the difference between MV Sea Princess and MV Grand Princess (see my first post in the thread "Sea or Grand? Caribbean or Hawaii?"), so I won't repeat all that here. My own opinion is that MV Sea Princess and her sisters are absolutely tops. About the only criticism of these vessels that seems to stick is that the cabins are somewhat smaller than on other Princess ships, but they certainly do have plenty of space and storage for two people and heads that really are functional. Given a choice, I would pick MV Sea Princess over MV Grand Princess any time.
The ports of call on the Western Caribbean itinerary are quite diverse. Here's a quick run-down:
>> Princess throws a great "beach party" at Princess Cays (yes, it's plural -- there actually are two islands connected by a short bridge). You can rent all sorts of "beach toys" like wave runners and paddle boats, there's a natural reef for snorkeling, a volleyball net, a combo playing island music and a fantastic beach for swimming (yes, the water is most wonderful!) and tanning. Before passengers go ashore, the ship's food and beverage department take ashore about two tons of provisions for the barbecue lunch, which is both excellent and included in your cruise fare. One of the islands also has a couple souvenir stands and a gift shop where you can make purchases with your cruise card even though they are staffed by locals.
>> Ocho Rios is a port where it's best to take a tour from the ship rather than going off on your own. If you opt for the latter, you will be accosted by many pesky taxi drivers soliciting your business. If you have not been to Ocho Rios before, I highly recommend the tour of Prospect Plantation -- which provides a fascinating showcase of the island's dramatic topography (at over 6300 feet, Jamaica's "Blue Ridge" is over 20% higher than Mount Washington), and a fascinating diversity of crops grown at various altitude zones. This tour also includes a stop at the Dunn's River Falls, so be sure to wear your bathing suit under your steet clothes and to bring a towel from the ship and a pair of SurfWalkers for the climb, which is most refreshing. The falls are well shaded by trees on both sides, so you'll be able to do the climb -- which is most refreshing -- even if you got a bit too much sun at Princess Cays a couple days before. You might want to pick up a couple pounds of Blue Mountain Coffee at the tour's brief shopping stop. (Yes, it's legal to bring roasted coffee beans, either whole or ground, back to the States.)
>> Grand Cayman offers several unique attractions including Hell, the sea turtle farm, and Stingray City. The island's Seven Mile Beach (which actually is only 5 1/2 miles long) just outside of George Town is one of the best in the world and George Town is a free port with some of the best shopping opportunities that I have seen anywhere. Many of the shops there seem to stock quality merchandise rather than the usual junk that you find in the shops of many ports of call. Although there's no problem going on your own on Grand Cayman, a tour from the ship will maximize your sightseeing opportunities. The "sea and see" tour, which combines a short cruise in a "semisubmersible craft" to view reefs, wrecks, and marine life under water with a land tour to Hell and the sea turtle farm, is exceptional, and Stingray City consistently draws rave reviews from passengers who opt to go there.
>> Cozumel has great beaches, but it's also the gateway to an encounter with the Mayan culture. The shore excursion to Tulum is very worthwhile, but it is a long day. If you opt for this tour, be sure to bring a big bottle of water from the ship for each person and to drink it often because Tulum tends to be very hot. If you want something less ambitious, the tour that combines the Mexican folkloric show in San Miguel de Cozumel with a visit to the Mayan ruins at San Gervasio (which is on the island of Cozumel) is an excellent choice. This Mexican folkloric show is one of the best that I have seen. Although the ruins at San Gervasio are not as extensive as those at Tulum, they are much more significant because the island of Cozumel was a holy place in the Mayan religion.
Overall, I doubt that you will be disappointed by any of these ports of call.
Do have a great cruise!