Would $100/mth be better off in an RSS plan (DBS Invest saver) or a high-interest savings account (i.e. SCB Jumpstart, CIMB)?

POSB Invest-Saver at $100/month is investing but SCB Jumpstart/CIMB FastSaver at $100/month is saving. It's not a matter of which is better. It's a matter of what are you trying to achieve - increase your investments or savings? If you cannot work out the difference, I recommend you put the $100/month into SCB Jumpstart/CIMB FastSaver as savings until you are ready to invest.

They're two extremely different objectives.

One is savings and the other investment.

So the question is better how? A savings account is guaranteed return while investments give variable returns and your capital can decrease.

So if this is the start of your financial journey and will be your first 100/mth saved, stick to a high interest savings account until you have your emergency funds settled, and then look at investing anything above that.