I think April is better. It is still cold over there in March , although still very cool in April, warm at noon. You can travel around without joining a tour. Then you need to know some preparation online to book the hotel and tickets online in advance, get a general idea of the car rent, taxi fare in Jiuzhaigou. If language is not a problem, that is quite well.

We were there late March '09. Expect mildly cold temperatures. Jiuzhaigou was beautiful, the wild cherry blossom trees in full bloom. Huanglong - not so scenic until after April when the coloured pools are filled and the postcard-picture beauty can best be appreciated. Easy to travel on your own - taxis are plentiful in populated areas. We hired a taxi driver for our 3 day stay. A little more expensive, but beware your driver takes you to buy traditional Chinese herbs like cordyceps - the shop he highly recommended sold us cordyceps "padded" with sticks! Expect to pay tourist prices for lousy food when eating out at JZG restaurants. High season rates apply at hotel and attractions from April to October.