haven't done domestic F in either but my experience is that Air China's domestic service is pretty subpar (lots of yelling from the stewardesses, improperly heated -if even- meals, etc). Shenzhen Airlines on the other hand, while by no means fancy, was much more pleasant to deal with - stewardesses smiled, spoke in polite terms, etc. The mainlanders I know consistently say that Air China is the worst since it's the flag carrier and doesn't have to care about customer service.