As a newbie in investment, what's the difference for the STI ETF and Phillip Sing Income ETF? Which is recommended or should I invest in both?

The technical difference is that the STI ETF tries to replicate the SPDR Straits Times Index ETF, when the Phillip Sing Income ETF tries to replicate the the 30 stocks Morningstar® Singapore Yield Focus IndexSM.

Singapore, however, even if sympathetic to You only a tiny fraction of the world stock markets, and - let's be honest - an unsuccessful one over the last 10 years when compared to a SP500 ETF (VUSA) or MSCI World ETFs (EUNL) when You look at this 10 year performance chart (dividends however not included):

And the Phillip Sing Income ETF is actually a very new one, and also a very small one with total assets (AUM) = 44.7 Mio SGD.

Funds with less than 50 Mio USD, the current thinking is, are prone to closure because of non-profitability to the issuing company. No good idea ...

You could read more, on particularly what not to do, here, good luck!:

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