Pardon me if my post gives you too much information to digest.
Chinese airlines are nothing like SQ (Singapore) or MH (Malaysian). The quality level is markedly lower, especially compared to SQ. However, they are all safe carriers which will take you to your destination safely and hopefully without too much trouble. The main thing which distinguishes the 3 carriers is their main hub, which will be your transit point. CA (Air China) is based out of PEK, MU (China Eastern) is based out of PVG and CZ (China Southern) is based out of CAN. Of all of these, CAN is the closest to the traditional Kangaroo Route transit points of SIN, BKK and HKG. Transiting CAN is also relatively easy compared to PEK. PVG is the most modern of the 3 airports but MU seems to have the worst ground handling. In terms of food, entertainment and service, all 3 are pretty much the same with edible (better than US-based airlines) food, poor/non-existent in-flight entertainment and acceptable service.
CA has the best seating, assuming you're flying economy, they have 34" pitch (compared to 31-32" industry standard) in economy on most of their fleet. If you're flying business, I also think they have the best business class (other than for CZ A380) of all the Chinese carriers. CZ has some great planes in their new fleet but they also have some planes with a bad layout like the 10-across (vs 9-across industry standard) rows of death on their 777. MU seating is not too much different from CZ, but I don't think they have any rows of death.
Another option to consider among the 3 is CZ premium economy. It's certainly a bit better than economy and the price usually isn't that much different. Legroom is much better, seats are a bit wider and you don't have to share overhead compartments with quite as many people.
I'd rather the 3 choices as follows:
1. CZ, for the option of cheap premium economy and convenient routing through CAN. (non-777)
2. CA, good standard seat pitch and the most "Westernised" of the 3 airlines imo.
3. MU, relatively poor technology and ground handling but a chance to eat some good food at PVG during the layover.
4. CZ on a 777 in economy. Rows of death.