I don't know how many people u have in your group. If it's just say two of U, I'd recommend booking a ship's excursion.
If there's a couple more with u I highly recommend a tour we've done twice, that we pre booked on our own.
We booked at <http://www.portpromotions.com> , and were met at the pier by our guide with a mini van just for us. We toured the island, and climbed the falls. The guide and driver were fun, and took great care in keeping the sometimes aggressive locals at a comfortable distance. Had a great day both times we did this!
If this is your first trip into Jamaica, I'd go with the tour, and yes they take you to a few shopping spots, we also went through fern gully. Its best to stay with a group while there, but if you feel comfortable with taking a cab, that is also a good idea.
Just make sure before you leave to have the amount of the ride set before you get in the cab and enjoy the falls.