which is better - bus or plane from Chengdu to Jiuzhaigou?

Both these two ways are convenient for you:
1. The direct flight spends around an hour on the way. Then hire a private car or take shuttle bus to get to the scenic area. The duration is around 1.5 hours.
2. If by long-distance bus, Chadianzi and Xinanmen Bus Stations has many buses to go to Jiuzhaigou. It takes around 8 to 9 hours on the way.