with the butterfly farm I would use Disney just for the ease of not worrying about getting back on time. it is a bit farther to go. If your on a disney excursion and you run late the ship will wait, but if you go on your own, they will NOT wait!!!
Coral world is fairly close to the port, about 30 minute I think and it is really easy to get a taxi to take you there if you want and to bring you back
Last year I think it was $11 per person one way , plus admission into the coaral world park itself, not sure on the price.
If you do either one on your own, go early morning so you can get back to the ship with a few hours before you have to. You can always shop near the port after an excursion if you want to so you dont have to get on the ship ASAP after your excursion.
i guess the advantages of doing
DCL excursion is security for time.
advantage of going on your own, it is usually a little cheaper and you can stay as longer if you like. taxi's are easy to find to go back and forth.
always set the price per person before you get in the cab!!!!
Personally , our first time to any island I always book with Disney because it takes all the worry and stress of being in a strange place away and I always feel safe with a large group and a guide.
2nd time to some islands I would feel ok going on my own.
mom x4,grandma x4