15 days (Beijing, Datong, Pingyao, Xian, Shanhaiguan)

Hi,

I am planning my backpack of 15 days trip travelling start/end at Beijing that includes other places such as Datong, Pingyao, Xian and Shanhaiguan. Any suggestion which route is the best:
1. Beijing=>Datong=>Pingyao=>Xian=>Beijing=>Shanhaiguan=>Beijing
2. Beijing=>Xian=>Pingyao=>Datong=>Beijing=>Shanhaiguan=>Beijing

I want to save time and cost by taking overnight trains between cities. Also what is the best breakdown on the number of days in different cities?
Btw, the period I am planning is during Labour day, which I know clashes with the holiday crowd (to leverage on the off days from work also at my end). So need to think which city with less crowds should I be when it happens.
Thanks much.