Hi Tar,
I would recommend IG - their HSI prices are mostly 24 hours which is great. I used to trade with City Index but they stopped offering 24 hours HSI trading during last year's market volatility. I do feel IG's HSI prices can be overly corrolated to DJ and S&P overnight, especially on the bearish side when S&P closes lower. Hong Kong investors are generally more bullish and they tend to shrug off things when market opens.
Hope this helps!