your on board the ship experience is going to be the same. Since both cruises have 3 sea days that will be similar.
You have really 3 different things to consider:
1 - travel time to the ship and home - as a fellow west coaster (Vancouver) it is a significant factor for me, if I can extend time out of Florida I don't object but it is a long way just to go on a cruise. The other part of that is cost - for me I can get cheap flights to LA but not so much to Florida.
2. weather - the two coasts have different weather - less of a factor in December but still a consideration
3. the ports - this is the big difference - Mexico ports and many Carribbean ports are a very different experience so it depends what you are looking to do. I have done the Mexican Riviera cruise 3 times now and so for me it is about relaxation time, but I still do a tour to the arch and spend time at lovers beach in Cabo, and do some shopping and exploring in PV, Mazatlan for some reason I have always done either hiking or food type things. On the Carribbean Princess Cay (private island) is relaxation / beach time. San Juan is a neat place and I would explore the forts (although I have been 2x so I might look for something new), St. Thomas has a lot of water (sailing, snorkelling) and some amazing beaches.
Either will be great.