Welcome to flyertalk lendbz
You use your HKSAR passport to enter Hong Kong and China and you use your US Passport for returning to the US. When you leave China use your HKSAR passport and for entering the US you user your US passport. I mean you have to use your US passport to enter the US or do you have a visa in your HKSAR passport?
It is always recommended for persons with dual citizenship to use the passport belonging to the country you are entering and a citizen of.