Hi Cara,
Per Disney Cruise Line policy, Guests are allowed one carry on and one purse or briefcase per person, and two checked bags per person. The size of the carry on luggage would be similar to what you are allowed to bring on a plane. If you have several pieces of luggage, you may want to put swimsuits, and a change of clothes in your carry on and check the rest of the luggage. Luggage delivery is usually very timely, but sometimes I've seen them delivering until 6 or 7pm, which means if you have Main Dining you will be in the same clothes you boarded with. That fine, because attire the first evening is cruise casual.
Between the two, I would definitely go for the
Coral World and Butterfly Farm Port Adventure in St. Thomas. There is so much more to experience on this tour. The Coral Reef that you get to observe is beautiful, and you have some time if you decide you want to go swim at the beach. I think it's definitely the better of the two.
I hope this helps Cara. Please let me know if I can answer anything else for you. Have a magical trip!