San Diego weather in December can be great - sunny days, cold nights. But, it can, and sometimes does, rain. It's just not as guaranteed-warm-and-sunny as Hawaii. Lots to do here though.

It also really depends on the kind of trip you want, even if it's warm and sunny in both places -- San Diego to me is more of a get out and do/see things, but in Hawaii, I can just sit by the pool/beach and read for a week (and be very happy doing that!).

December is not hang out and play at the beach weather in San Diego, unless you're a surfer with a wetsuit. It could be sunny, and it could be cloudy/rainy. However, as stated above, there's lots to do. As a Seattlite, you probably want warmth and sun, so Hawaii seems to be your best choice.

Hawaii...there was no pause to think about it either.

Ok, Hawaii it is.

Yay!! Now, do you need help with lodging recommendations?