Both the cities are good to visit in mid December. if you planning for leisure trip then I would suggest Udaipur is best. The climate is pleasantly cool and the temperature remains comfortable. So that you can visit the monuments and nearby places of udaipur. While Kolkata is one of the most tourist friendly metropolitan city. Most of the people going there to celebrate Christmas & new year festival.
Udaipur is cold and chill all through winter with blend of pleasantness. At the same time Kolkata is a beautiful must visit city to visit all through December. I would suggest you to plan both cities visit as per your travel plans.
Both are Worth Visit Cities. Udaipur is best destination in Rajasthan famous for Lakes, Palaces, History nd Culture. Kolkata is City of Joy, Big Metropolitan City of Bengal famous for Art, Culture nd Heritage
Udaipur is surely charming. Days are chilly, but there's enough sunlight too, though nights are colder. Kolkata too is an excellent place to visit during the winters, but my favourite is Rajasthan in the winter season. Happy journey 🙌