Air China (CA) and China Eastern (MU) are pretty comparable, as mentioned above, in terms of the service levels. I think the only other possible distinction to choose one or the other is your aircraft preference.

CA flies Boeing aircraft - long hauls are 747.

MU is mostly Airbuses - A340s, for example.

Flying domestically in China I don't have any aircraft preference as long as it's a modern Western aircraft and not an old Soviet relic.