You don't make the choice easy.
Freedom and Dream are arguably the best ships Carnival has.
I personally like Jamaica and think it gets a bum rap.
Grand Cayman has wonderful beaches and Stingray Sandbar - one of the best shore excursions, anywhere.
Cozumel has a number of beach clubs. I love to snorkel at Roatan. There is a (nurse) shark-ray snorkel available in Belize.
Both Grand Cayman and Belize are tender ports.
Sometimes Grand Cayman is skipped; sometimes Costa Maya is skipped.
The Dream itinerary is Sea Day, Port, Port Port, Port, Sea Day. Some people don't like 4 ports in a row - it can get very busy (but how hard is it to plop down on a beach?)
Freedom is Port, Sea Day, Port, Port, Sea Day.
Some people think Dream is too big, but then some people also think Freedom is too big.
As for what happens if you hate it? Carnival has a vacation guarantee, but make sure you understand it - you have to decide and let them know before the first foreign port of call (Key West isn't a foreign port)
Carnival Vacation Guarantee | Carnival Cruise Lines
I would be happy with either. So there you go! Clear as mud?