FO!
You will get frustrated sitting behind the action as a SO. Your salary is higher as a DEFO on UFO scales(CoS08) than it is as a SO in HKG on CoS99.
You can always bid across to pax fleet in 4 years.
Tax is a separate issue. If you plan a career at CX you will have plenty of time to buy a house. You will have more of a deposit earning FO salary for 4 years than you will in HKG as a SO. Cost of living is very high here!
Just my opinion though!