I am a beginner planning to invest a starting amount of 2k instead of leaving it in a bank account, I am also able to set aside about $100 every month. My question is go for STI ETF or Robo-Advisor?

Robo for me

Short answer: Choose the roboadvisor. Assuming you're a young adult, capital appreciation is probably the most important for you. Going through a robo-advisor, which can invest in global/US equities is probably a better bet for you!

Given that you just started investing, I highly recommend you going with Robo-advisors with a global portfolio. This will give you a more diversified portfolio and greater growth potential.

I personally have investments in 3 of them namely Syfe, StashAway, and DBS DigiPorfolio.

I do monthly updates on my portfolio returns so you can take a look here.

Robo-advisors. Look into Stashaway, Syfe or even Singlife or Autowealth.