You will most likely be hauling all those heavy luggages up and down the stairs if you use the K trains. The "old" stations that don't serve bullet trains are a bit of an experience to use.

What is your trip purpose for Chengdu? If only for the pandas (and there isn't much else there worth seeing - rather use the time to explore Xian), skip it as you will find pandas in Beijing or even in Hong Kong.

Xian was China's imperial capital for 1000 years. Give it some justice.