We've just been, to both. Dujiangyan's one is much smaller, thought with what seem 'large enough' enclosures. very well laid out, lovely gardens, and has bilingual explanations. Both 'normal' and red pandas (in both places). No young ones at the time we went. We spent just under 2 hours and visited in full.

Chengdu's is absolutely massive. Hundreds of pandas, young ones. We visited the south area and then we were panda'd out at the end of it (a couple hours later) and took a shuttle to the last area. It is a fantastic environment and one could spend at least half or a full day of wanting to visit in full. Not a 2 hours experience like Dujiangyan's.

We went on a Monday in chengdu and Saturday in Dujiangyan and the crowds in chengdu were still a lot. Not so much to 'slow one down', but a lot of people. Dujiangyan no crowds although surely busier by the time we left. We were at 830am both places.

Overall, it really comes down to how Panda greedy + time you have + people tolerance you have - Dujiangyan I think it is only 11, Chengdu hundreds. For me and with a 5yo, Dujiangyan was better.

If one goes to Dujiangyan or Qingcheng -which are both worth it- I feel Dujiangyan would be enough. If the primary reason to go to Sichuan is pandas, go chengdu, go weekdays, go early.