Have been to Cabo several times, but not in December. Have been going to Puerto Vallarta for 30 years (a couple of times in December, when I usually stay nearly a month) so can't really speak for Cabo in December but PV has a very welcoming vibe, in the local community, in December and the weather is perfect.
You cannot beat Puerto Vallarta for a wonderful relaxing vacation in December. I was there for a month from Thanksgiving to Christmas last year. Couldn't have been better, with weather and food and activities and just feeling you're really part of Mexico, rather than staying in a resort.
Personally, having been to Cabo several times, it would be the last place on my list that I'd want to spend December ---- and I own a timeshare there (which I usually trade into PV).
I usually trade into Puerto Vallarta through RCI timeshares or stay in a Mexican hotel but if you're looking for resources as to where to stay (I've stayed in two condos in PV for a month each but they are no longer available to rent), you will get excellent suggestions here.
Come to Puerto Vallarta in December. You will love it!