Go both, that is my suggestion! The only month I have not been to the Caribbean is July, and been to SXM all but July and August. June even in the Pacific NW is usually decent weather so not inclined to go to the Carib then, December however is a nice break from our weather but that factor depends upon where in California you live. My favorite time to be there is probably late November to early December, business is getting going on island as season is about to begin and the staff is not exhausted yet! We also have a pretty fun group that is there around that time every year so that factors in pretty big. That noted we have never had a bad trip there although September was pretty hot and humid.
Have a great trip!