I would look at the itinerary and see how many hours you have in each port. If you are only in St. Martin from, for example, 7AM-3PM, you might not get much partying in. I hear Barbados is a blast.
I am also a partier in my 20s and we are going on the Destiny to St. Thomas, Aruba, Dominica and Barbados on January 9, 2005. I wanted to have some later nights in the port and this itinerary is great - in Aruba from 8AM-11PM and Barbados from 7AM-10PM. Can't get much better than that.