I can't decide between Shanghai and Zhang Jia Jie.

Well, I'm afraid that December is cold for touring around Zhangjiajie Scenic Area. The scenery is not that attractive and it can be very cold. So you are suggested to visit SH. The temperature may range from 35 F to 50 F in that month. But there will have many sunny days. It won't influence your sightseeing in downtown area. About the clothes, you can prepare jackets, thin sweaters and pants.

Thank you for your prompt reply. So we are now set for the city!