Overseas passport has more advantage？

Both parents are Hong Kong people with Australian passport and speak cantonese at home. When we apply IS for our son，should we use Aust passport or HKSAR passport？ which one has higher chance of getting in？ The issue is if policy is say for example to take 30% local student and 70% foreigners， seems like applying with australian passport is better， but then we have to compete with the native english speakers？will your answer different for different schools？