Neither will be summer-like weather. Might be chilly and inclement, might be sunny but cool.
The people in both locations seemed friendly to me, but perhaps the people in Greece, as compared to the South of France, were a bit more down-to-earth and unpretentious.

Cil is right, it won’t be beach weather in any of these places. South of France will probably be quite cool — it actually does snow on the Riviera once every couple of decades. Costa del Sol in Spain will probably be the “sunniest” in that the sun shines for at least a few minutes almost every day. However, it’s considered one of their 360 (or whatever ludicrous advertisement you see) sunny days if the sun shines for 10 minutes. Greece may be warmer, but some of the islands are strictly summer resorts and all but shut down in the winter.

If you’re looking for the typical “beach party for singles” I would not go to Europe in the winter.

Go to the Carribean.

The Mediterranean is not warm this time of year.

I would love to go to the carribean but I want to see Europe and this is the only time I can do so!

It’s definitely still worth visiting but you won’t be able to do beach stuff.

Newyorker:

It’s all good, just don’t expect beach weather, and the social atmosphere that comes with it. You can certainly orient a Spain, France, or Greece itinerary for December. Just don’t emphasize “summer destinations”: places like Mykonos, Ibiza, etc. It’s also important not to put all of Greece’s islands into the same basket. Some of the smaller ones, which are summer destinations, will be quite dead in the winter (so, avoid Mykonos, Ios, etc), but the larger more populated ones (like Crete, Rhodes, and Corfu) are still worth visiting, for their architecture, historical sights, and festivals (if you’re there during Christmas).

You’ll have to decide: If the beach social atmosphere is more important to you than actually sightseeing in Southern Europe, then consider the Carribean or Southeast Asia, both of which are in-season in December. Mediterranean Europe is in-season from May to September, with July/August being the height of the season.