HKU CS offers two kinds of research degree programs, PhD and MPhil. If you are a BS student debating which one to pursue, below are my opinions about their relative pros and cons.First of all, I assume your goal is to publish in top venues regardless of your choice of the degree program. If not, you should not approach me as I will not be a suitable mentor for you. (See here for more details.)
Under the above assumption, my quick summary is that MPhil has higher risks and higher rewards, while PhD has lower risks and lower rewards. This is based on my personal experience in that a good BS graduate will take around the third year to start publishing top papers with me. (My criteria for being *good* is that you can make internship with Microsoft Research Asia or Redmond.)
Personally, I have no strong preferences for either, as long as you can have top publications upon your graduation. You will need to decide which one is better for you based on your personal situations.
PhD
The duration is about 4 years, so you are more likely to have top publications upon your graduation (lower risk). However, you will be a HKU, not Stanford, PhD (lower reward, even though I would say HKU beats any non-top-25 US schools).Note that I will not sponsor any attempt to drop out of a PhD program in order to apply for another school regardless how well you have done, as obviously the department would frown upon any such (bad) precedence. You need to man up your own decisions.
MPhil
The duration is 2 years and mostly non-extensible. If you can manage to have top publications, I will be more than happy to recommend you to a top US school (higher reward) if that is your goal. However, chances are that it might not happen (higher risk).I will be happy to help you whatever I can, but 2 years is a very short period of time for me to change someone dramatically (I am not god). So, if you do not think you can even make top 40 US schools after BS, it is not very realistic to expect making a top 5 US school after a 2-year MPhil. (Your parents, not me, are responsible for your development prior to the BS.)
However, even without making to a top 5 US school, you still have a chance to continue your PhD program with me, if you can do sufficiently well during your MPhil. But the risk is there; under the same (under) performance, you will have another 2 years as a PhD, but I might not take you back after your MPhil.