Would it be a better choice to do DCA in STI ETFs or S&P500? ?

I agree with Nicholes, S&P500 has made a good rally post reccession as compared to the STI. Just by looking at the chart, it the STI didn't seem to move much at at all, whereas the S&P had the historically longest rally since 2008. Buying to the S&P might be expensive now, especially if a potential recession is coming. So just be more careful on that.

Also, a DCA strategy may not make sense during a recession. Since the markets are going downhill during a reccession, you might be better off postponing your investments.

S&P 500. As they have better returns and more diversified. You can also consider global ETFs like IWDA or VWRD for global exposure.