We did the Western in March and had a good time , but if the ship stops in Costa Maya, stay on board. Cozumel is interesting, at least from my POV because I live diving and snorkeling and Coz was some of the best I have seen. As was Grand Cayman.
However, if you want a lush "TROPICAL" experience, the Western Carib is not it. We have done 2 Southern cruises and are thinking about doing another next year. The islands are more varied, the people friendlier (at least in my opinion) and for me there is so much more to do and see.
The weather in Dec will be about the same for either destination, except that if you leave from San Juan (for the Southern), it will be warmer than Fort Lauderdale.