In my point of view, local school is more stressful than IS, IS provides an open-learning environment, at the same time they're not rushing for the exams, they ensure every student understand the topic, so less (or no) after school tutorial is needed. This is one of the reasons why I put my girls in IS.
But of course, you have to bear in mind that their Chinese standard may be lower than those local school students. Though, I'm satisfied with their Chinese standard.