What I meant by HKU>CUHK>HKUST was on their general reputation. For maths, the ranking in HK should be CUHK>HKUST>HKU. The CUHK I am referring to is The Chinese University of Hong Kong. Professor Shing-Tung Yau is in charge of a mathematics institue in The Chinese University of Hong Kong. His charisma attracts talented maths people of Hong Kong and from Mainland China to study in The Chinese University. Stephen Smale is also in Hong Kong. He is the University Professor of City University of Hong Kong (CityU Hk), which is considered by most HK people as one of a second tier university in Hong Kong.
I understand the OP wants to pursue a career in Finance. Unlike prestigous universities in UK, universities in HK have developed specialised courses like Quantitative Finance, Risk Management, etc to attract maths talents with career interest in Finance to study. The scholarship now offers by HKU to the OP should be a general maths degree; this can be evident by the faculty based scholarship offered by HKU to the OP because starting from last or this year the Science Faculty of HKU started to offer places on faculty basis. If OP decides to study in HKU, he should check whether he has the chance to study financial maths because it is not sure those financial maths courses would be available to general students of its Science Faculty.