Maybe its worth visiting different schools (international and local) and compare teaching methods, school ethos, homework policy, breadth of curriculum, opportunities to shine in other areas as well as academics, music, sport, etc and see which one suits your child. You may need to decide which of these things are important, then choose a school that meets your requirements. Different children thrive in different types of school - there's no hard and fast rules. A friend of mine has a child in a local school and has another in an international school because that works best for her children. They are both thriving in their own way and the main thing is that they are happy. Of course, the older child is completely bi-lingual in every sense but the younger perfers to use English more.
I think if you send your child to a international school, a couple of hours of mandarin/cantonese is not enough for them to grasp hold of the language adequately (so that they are at completely fluent level), especially if this is not backed up at home. Of course, you could get a chinese tutor to help with language if that is important to you.
These are just some thoughts......