Is it better to invest in individual bank stock, or STI ETF?

Neither, nor.

Why STI ETF is a very unsuccessful product, single stocks are even worse and (almost) anything else that matters You could read on this 'caleidoscope' here:

https://seedly.sg/questions/what-is-your-genera...

Bank stock - Higher risk higher returns.
ETF - more stable, but less returns.

For me I do both. ETF helps to diversify and bank stock to get higher returns.