i'd say a n. american degree is the best if you plan to stay in the US after you finish your degree. also, if you plan to do a phd (presumably in the US), definitely get your masters in n. america unless HKUST is ranked much much higher although HKUST is the best university in HK.
i'm from HK myself. i can tell you what to expect if you go to HKUST.
HKUST is really isolated. expect to go out and hop bars only during weekends. people in HK are nice and friendly, and the city never sleeps so you won't get bored when you're outside the campus. however, expect to compete against lots of students from mainland china who have very strong technical skills and have no life aside from studying. and right, my guess is, most of your instructors will be chinese. what i'm trying to point out is, if you're not chinese, expect some cultural shock.