We love the Southern Caribbean. Its a very port-intenstive route, lots of variety in the ports, its not as commerical(for lack of a better word) as some of the other routes, more local flavor.
San Juan is great, fly in the day before, San Juan airport is zooey at best when all the flights to get to the cruise ships come in. If you can stay an extra day on the end of your cruise that would also be a good thing.
Lots to do and see in San Juan.
My personal ship choice would be one of the Voyager Class ships of RCCL.