AC vs MU
I flew Air China LAX to PEK, 2am departure last May in coach. Service and comfort was OK. By comparison, a NWA flight from SFO to NRT was worse on both comfort and service.
I flown a number of AC domestic segments with no problems.
I flew China Eastern (first and only time) from Kunming to BKK in coach in June. Service and comform the same as the AC experience (no problems)
IMHO, the two airlines are very close as products. Take your pick!