Hi, if u want to pass the HK immigration dept fast, u are better to use ur smart HKSAR ID card. Of cos u also can use ur HK passport to pass the HK immigration dept too.
ur HK ID card is using for enter HK only. when u are check-in in USA, some airline agent may ask u to present ur HKID card. if u are holding HK passport and HK ppl, u can tell her u will use the HK passport to enter HK.