Northwestern Econ is known by everyone and anyone in the employers' world here in HK. If you can get the Kellogg Certificate even more so.
Anywhere between 60%-90% of people reading resumes for top business jobs in HK (Consulting, iBanking, etc) are US educated, so they will all regard Northwestern as very prestigious.
Comparing Northwestern and HKU is a joke lol. Layman's reputation for Northwestern in HK is, of course, less than spectacular since the average joe will only know HYP, Oxbridge and HK universities at best. But if you're concerned about day-to-day prestige I can't help you - honestly nobody gives two damns.
But in the job market and for people who matter (i.e. educated people in HK), there shouldn't even be a comparison between Northwestern and HKU. Northwestern, even disregarding the fact that employers here are biased towards foreign degrees, is a MUCH better institution all around. Better teaching, better funding, better facilities, better social life, better and closer alumni network. Uncle did an MBA at Kellogg and he got in touch with the Kellogg alumni network here to try and find me a job; Northwestern has a huge network here in Asia. Better than HKU honestly, even though it's like 200 times smaller.
And one more tip: Recently I was networking, trying to find a job this summer. I will quote
verbatim an informal text conversation I had with a banker at a top bank (the type of company that all HKU business students would get their panties' wet over): "We mainly hire non-local grads because most local grads are useless but an American studying in HK would be worth a look."
There's clearly a huge (perhaps justified) bias against local HK grads. The common string I've pulled together from conversations with a lot of hiring professionals is that
the best HK talent goes overseas to study. This is a fact - on average, the best and brightest get the **** out of this place for 4 years, have more fun, meet nicer people, get a more outward-looking worldview, build a more interesting personality, and come back to dominate the other HKers when it comes to job-hunting time. Nobody likes to admit it but this really is overwhelmingly the case. Even though you were bright enough to get into Northwestern, if you choose to go to HKU, there
will be a generalization that you are not as good as a foreign grad - not just academically, but
as a person. This perception is here to stay for at least the next 20 years, since all the top jobs hire US/UK educated people, and those people in turn have the same prejudices when they hire applicants.
See here for anecdotal evidence. This is a girl from CUHK bemoaning how tough it is to compete with the US educated grads in Hong Kong, even though she's probably just as bright:
http://www.wallstreetoasis.com/forums/too-many-americans-us-graduates-in-hong-kong